Three versions exist, A, B, and C.
A was used primarily in the 400/800 computers, B in the XLs, and C in the XEs.
Aside from being built-in in all the different machines, rev B and C carts were also made. To determine what version you have enter PRINT PEEK (43234). Rev A returns a value of 162, Rev B is 96, and Rev C is 234.
Rev A was a beta version from SMI (Shepardson Microsystems, Inc.), the company Atari contracted to write Atari BASIC. As a result of trying to fit the program onto an 8K cartridge, as well as working on a tight schedule, Atari BASIC was plagued by 2 problems that caused it to be one of the slowest-performing BASIC languages for any computer – a poor implementation of line number lookups in loops and jumps, and a poor implementation of multiply and divide. Unfortunately, Atari never corrected the problem in subsequent versions, even though SMI identified and fixed a number of bugs (some serious) that cropped up in the initial release.