I addition to Nikola Djordjevic's answer, you can also use the
todo package, with which you will be able to write todos in your output document, plus print a list of todos in your document at an arbitrary location. Nice gimmick: use the option
disable to ignore any
todo commands in the document when you need to pass on your current work to someone else and you don't want them to have to look at the todos, but also don't want to go and comment each one out.
Huge plus for me: TeXStudio will also recognize
todo commands and highlight them and their content, not just the todo comments as shown in Nikola Djordjevic's answer. This way, you can have both, source code comment style todos for just the people with access to the source file and command todos included in the document for todos you want to see in the PDF (or on paper). In TeXStudio, both styles will be included in the bookmarks in the left part of the program window.
Additionally, in TeXStudio, you can go to Options > Configure TeXStudio > Adv. Editor and tell TeXStudio which keywords you want it to highlight as todo comments. (Check the checkbox "Show Advanced Options" in the lower left corner of the configuration panel to access the "Adv. Editor" tab.