From list of obsolete packages and classes we must avoid, I found 2 terminologies, i.e., font-encoding and input-encoding. What is the difference between them?
Input encoding is on the, sorry, input side, i.e. getting the characters from your input file into (La)TeX correctly. Font encoding is on the output side, i.e. "I want to print an 'A', where in my font do I find it?".
A file stored in your computer is just a sequence of numbers in the range 0–255. When you give a file as input to a program, this program has to know how to interpret this sequence of numbers. A text editor will interpret the file by associating a certain glyph to each number (or subsequence of numbers) based on well defined rules, which I'll call "code pages".
In the olden times the allowed numbers were in the range 0–127 and there was only one code page, ASCII (actually some systems used another method called EBCDIC, but this is not very relevant). Of course such a limited range made impossible to represent the necessary characters for languages using accents, diacritics or alphabets altogether different from the basic Latin alphabet. Therefore many code pages were developed, filling the range 128–255 in different ways.
TeX uses a different interpretation than text editors: the numbers it reads are subject to the tokenization process, so that control sequences can be distinguished from normal characters. But at the end, TeX typesets characters and when TeX is told to "typeset character number x", it outputs the glyph it finds in position x of the current font.
Original TeX (version 2) could really understand only input in the range 0–127, but could manage fonts with 256 glyphs, for some sequences of numbers could be mapped to glyphs via information contained in the current font metric file (this is still how
The LaTeX development team therefore devised a new strategy. You can announce LaTeX what code page you're going to use for input (say
The method works by providing an intermediate layer, called LaTeX Internal Character Representation (LICR). Each input character is changed, based on the information given at the start with
to a LICR; for instance, the number corresponding to
Having a limited number of output code pages is necessary for avoiding the need to develop font files for each input code page, while maintaining the possibility of correctly hyphenating words: TeX is able to hyphenate only words formed by characters in the same font and not cointaining "built up" glyphs, such as accents over base characters.
The situation changed a bit with the advent of Unicode and UTF-8, which is a method for representing Unicode with sequence of one up to four numbers read from a computer file. When you say
you're essentially free from the "only 256 characters" limitation for input encoding, but the limitation is still present for output encodings. Thus you can't expect that LaTeX is able to interpret correctly mixed input using, say, Latin, Cyrillic, Devanagari and Chinese without properly segregating each part in order to use the right output encoding.
For example, a document written both in Italian and Russian will use the
the base language will be Russian and the default output code page will be T2A (automatically selected by
so that the Italian parts will use a 256 glyph font containing also accented characters.
If one says only
The editor you use for writing LaTeX documents is not TeX or LaTeX. You have to take care that the input mapping used by the editor corresponds to the input mapping used by LaTeX. There is no general advice for how to ensure this equivalence, because each editor has its own ideas about it.
The best advice is: always use UTF-8.
You need an input encoding to tell TeX how to interpret the contents of your text file, you need an font encoding for proper hyphenation. Old TeX can only hyphenate words from one font and therefore you need to squeeze all the characters you use (including all accented ones) into one font. If you need more than 256 different characters in one word you're out of luck.
So the input encoding is very important as a wrong input encoding makes it impossible for TeX to interpret the text correctly. The font encoding is not that important, as long as all your characters are represented. There is for example the T1 (== ec, tex256) encoding which is widely used, but there are others such as the original OT1 font encoding.
When you write a German text, you'll get to know input encoding and output encoding really fast. Assume you want to write the word
Let's assume TeX wants to use the byte "255" for the
Assume you use latin1 input (as it used to be standard on Linux). The word "draußen" is saved on your harddrive as bytes 100, 114, 97, 117, 223, 101, 110, and the
But how does TeX know what to do with the
so this is the magic part that leads TeX to choose the byte number 255 when it encounters a
The font gets re-mapped so the byte 255 is matched to the glyph named
It gets even more ugly
You can even make things worse by re-mapping the encodings internally by using virtual fonts. This is a common practice with the psnfss package. But I don't give details here because it does not help understanding the inputenc/fontenc subject.
In my answer on greek text I gave an explanation which I'll basically repeat and extend here. I'll be basically talking about text encoding here, as it is rather obvious that math requires special treatment.
Please also look at encguide where all this is explained in more detail.
First of all, all this refers to TeX engines like
This means the engine can only understand 256 different characters in input text and output at most 256 different characters in a given font.
For 16+-bit aware systems like LuaTeX and XeTeX, all this is irrelevant.
It shouldn't surprise that 256 characters are few to express all the characters there are, even if not considering chinese and such. encguide has some code tables, and looking at the table for T1 (which is the "standard" in some sense) it is obvious that it's completely stuffed and still some characters are missing. T1 represents the effort to squeeze in every character needed for european languages, but still it misses some special characters for polish or lithuanian, for instance, not talking about greek or russian.
Now, what is the strategy for producing documents which can contain more than 256 different characters? The answer, of course, is font encodings. Basically, this means
Note that when using a PostScript or TTF or OTF font which can contain more than 256 characters, it is not strictly neccessary to have different fonts for different encodings, TeX offers "virtual fonts" as an intermediary concept. But I don't know a thing about those ;-)
Back to encodings. What happens internally when I say, for instance,
How is the latter achieved?
Every font encoding is associated with a file
This means that for instance, the control sequence
so the polish Ł is in a different place in fonts having the respective encodings, but LaTeX will sort that out internally.
Basically, this means that LaTeX can internally handle any number of different text characters, but can output only those which are available in the currently active font encoding.
But where do the control sequences like
Of course, you can always type them as control sequences, but when you are writing a text in a specific language, you are of course expecting to be able to type the characters available on your keyboard directly.
This is handled by input encodings. For convenience, input encodings are organised based on the "code pages" which are (or were) usual on computer systems with 8-bit input scheme anyway, that is, ISO 8859-<something> (called
Typing text into a text editor with a certain code page active means that by typing a certain character, its associated code for that code page is written to the text file.
To declare a certain code page to LaTeX, there exists a file
Internally, this means the input character 198 will produce the control sequence
I hope this clears things up.