Sometimes considered acronymic for Practical Extraction and Report Language, although there are those (myself included) who think that's more likely to be a retronym. Usage supports this belief, since the language may be referred to correctly as either Perl or perl, but not PERL.
Perl is a scripting language similar to C but with more high-level functions built in. Also unlike C, Perl is interpreted, not compiled - but uses insane levels of precompilation and automatic optimization that can often make perl scripts actually more efficient at runtime than C applications. Some people have even gone so far as to write device drivers in perl, although that is a rather bizarre and not recommended usage.
Perl is intensely cross-platform - it is difficult to think of a hardware platform that doesn't support a Perl implementation, and a careful programmer can easily build very complex scripts or apps in Perl that require no porting at all to run on different platforms. It is also very flexible, and is used commonly for both CGI scripting for webservers as well as a tool to automate many system administration functions, and even to build complex applications. Finally, perl is extremely extensible through the use of Modules, which can take the form of anything from perl subroutines themselves to machine-level code, and which may be gotten from CPAN and built into the base interpreter.