I think the best description is given by the author himself in the documentation (1.1 About biblatex):
This package provides advanced bibliographic facilities for use with LaTeX in conjunction with BibTeX. The package is a complete reimplementation of the bibliographic facilities provided by LaTeX. It redesigns the way in which LaTeX interacts with BibTeX at a fairly fundamental level. With biblatex, BibTeX is only used to sort the bibliography and to generate labels. Instead of being implemented in bst files, the formatting of the bibliography is entirely controlled by LaTeX macros, hence the name biblatex. Good working knowledge in LaTeX should be suffcient to design new bibliography and citation styles. There is no need to learn BibTeX’s postfix stack language. This package also supports subdivided bibliographies, multiple bibliographies within one document, and separate lists of bibliographic shorthands. Bibliographies may be subdivided into parts and/or segmented by topics. Just like the bibliography styles, all citation commands may be freely defined. The package is completely localized and can interface with the babel package.
Instead of BibTeX (the program) or BibTeX8 (the 8-bit reimplementation of BibTeX), with biblatex you can also use biber as a backend. It can handle UTF-8, has advanced sorting capabilities and is intended to replace BibTeX.
You should consider switching to biblatex when you often have style guides for which you do not have appropriate BibTeX styles, or when you need to use UTF-8 for some reason. Some hints on what to take notice of when switching to biblatex can be found in the answers to the following question: What to do to switch to biblatex?
BTW: The documentation is excellent, and the package itself is very “mature” compared to other packages.