A very old interpreted programming language that was once popular with the masses, back in the days when Basic was one of the only Programming Languages that was usable by the general public at all. It is rarely used today, simply because there are far better languages around, though many still think of it with a tear in their eye and a song in their hearts.
Since the time it was first implemented Basic has been mostly used as a (arguably quite poor) language to teach beginners the concepts of programming, hence the acronym; Beginner's All Purpose Symbolic Instruction Code. However, BASIC used so many bad constructs (like the dreaded GOTO command, which led to so many instances of "spaghetti code"), that it soon personified bad programming practice and slowness.
For the beginner, the LearnProgramming page gives some good advice.
The BASIC language has regained some popularity with the Microsoft VisualBasicLanguage rapid application development environment and its use as an embedded scripting language in their Office products.
XBasic http://www.maxreason.com/software/xbasic/xbasic.html and RapidQ http://www.basicguru.com/rapidq/ are examples of cross platform UNIX/Win32 free or OpenSource BASIC compilers.