From the home page:
"...one of the most highly regarded and expertly designed C++ library projects in the world."
— Herb Sutter and Andrei Alexandrescu, C++ Coding Standards
"Item 55: Familiarize yourself with Boost."
— Scott Meyers, Effective C++, 3rd Ed.
"The obvious solution for most programmers is to use a library that provides an elegant and efficient platform independent to needed services. Examples are BOOST..."
— Bjarne Stroustrup, Abstraction, libraries, and efficiency in C++
So, it's a range of widely used and accepted libraries, but why would you need it?
If you need:
- regex
- function binding
- lambda functions
- unit tests
- smart pointers
- noncopyable, optional
- serialization
- generic dates
- portable filesystem
- circular buffers
- config utils
- generic image library
- TR1
- threads
- uBLAS
and more when you code in C++, have a look at Boost.