Part 1: What numbers meanHow do all these programs work anyway?!? (AKA what numbers mean)
There are lots of numbers in Decker, most of them referring to ratings. Program ratings, skill ratings, server ratings, IC ratings, hardware ratings, all kinds of ratings! But how do they work together to screw over you, the player?
With every program you use, even passive ones, you are making a d20 roll against a target number. If you beat or exceed the target number, you succeed; if not, you fail. To fuck with players (because more randomization leads to less benefit from strategy), on this d20 roll a natural 1 always fails and a natural 20 always succeeds.
So what are you rolling against? Well, the base target number is usually 10; a completely unmodified roll thus has a 55% chance of success. But the target number is never unmodified; that's where all the ratings come in! To calculate it out:
Actual target number = Base target number + (2 * System Rating) + IC rating - (Program rating + Chip rating + Skill rating)
Therefore, on the easiest possible server with the lowest starting stats, your target number is actually 10 + 2 + 1 - (1 + 1 + 1) = 10. Whoops! So you have a 55% chance of success on every action. Better than even odds! And for each rating point higher than the server in any category, your odds go up by 5%!
Except you're making this roll for basically everything you do. Moving past a Gateway IC? Roll against your Hide program, with a barely-better-than-a-coinflip chance of success! And that's just one test against one IC. Two Probes happen to cross patrol paths where you are? You'd better make that (.55 * .55 =) 30.25% chance of hiding from both, or at least one will query you and one or both will stop to hear the answer!
Now that's all just success or failure. Several programs (attack programs, Silence, Smoke, etc.) use degrees of success to figure out how much damage they do or how long they last. For those, the game looks at the difference between what you rolled and what the target number was, and pumps out a number for that degree:
+0..3 = 1
+4..7 = 2
+8..11 = 3
+12..15 = 4
+16..19 = 5
So if you're running Silence and succeed by 3 or less, you only get 1 second of Silence to do one action. Same with Smoke, the degree equals the duration in seconds. Attack uses double the degree to determine damage; succeed by 5, and you'll do (2 * 2 =) 4 damage to the IC.
What this all adds up to is that you're quite likely to fail missions when first starting out, and your ratings in all aspects should be higher than the server rating if you want better than 55% chances. Good luck, chummer!