If you use the “normal” libstdc++ shipped with g++, its documentation is available online here.
Most Linux distributions make it also available offline as a particular package; for Debian-derived distros, for example, it’s libstdc++-6-<version>-doc (e.g. on my Ubuntu machine I have libstdc++-6-4.4-doc installed). In general the documentation will be put somewhere like /usr/share/doc/libstdc++-6-4.4-doc.
This about implementation-specific documentation; for compiler-agnostic docs, instead, many sites on the Internet provide reference documentation for the standard library.
One of the most referenced is nowadays cppreference.com, that is actively maintained, tends to be very faithful to the standard and shows well the differences between the various standard versions; it can be a bit intimidating to newbies, though.
cplusplus.com historically was one of the most used (especially as it is very “liked” by search engines), but was known to contain several errors or incorrect simplifications; I don’t know if it got any better in these last years.
Also, the C++ library section on msdn.microsoft.com has got much better in the recent years in separating what are the Microsoft-specific details from what the standard dictates.
Finally, if you want precision up to the paranoia, the ultimate normative document is the C++ standard, that is sold from ISO, ANSI and BSI (for a quite high price); there are however several drafts available for free, which are more than good enough for “casual use”.