Programs are the work horses of Netrunning; they do the fighting, protecting, decrypting and sneaking for the 'Runner. If a Netrunner is a cybernetic magician, then programs are his spells, there at his mental fingertips.
Programs are rated by Strength, Class, Memory Units used, Cost and ICON:
Strength is how powerful the program is, relative to other programs. In combat, the Strength of a program is usually added to the Netrunner's attack roll (much like Weapon Accuracy in a combat situation). The higher the Strength, the better chance the program will be able to do it's job.
Class is the type of program; its function intrusion programs sneak in. Detection programs detect, Anti-IC programs attack other programs, and Anti-personnel programs attack Netrunners. And so on.
Memory Units represent the size of the program. All programs are measured in Memory Units, or MU. Each memory of a cyberdeck or system can hold 10 Memory Units. This means space is at a premium for Netrunners, you can only stack up so much in one run.
Cost is the price of the program on the open or black market. Nothing in the future is free. Not even the air, chombatta.
The ICON is what the program usually looks like in the Net. But don't count on it, you can alter your program's ICONs to suit your own tastes and style. Just goes to show; don't trust anything.
Enough talk-talk. Read the programs and spend your euro. You got a run to make.