First thing to know about Attack Speed: It means Attack Cooldown, not Attacks Per Second.
Second, after you understand the above, there's really not much to get.
If you have a base attack speed of 2, then get an item that gives +10% Attack Speed, you attack 10% faster. In other words, it will divide your attack speed (because if it multiplied then it would increase the Attack Cooldown thus decreasing the Attacks Per Second).
Attack Speed is calculated by: BaseAttackSpeed / (Multipliers). +X% is "* (1+X)" and -X% is "/ (1+X)"
e.g. you have a base attack "speed" of 2 and have a +10% Attack Speed, another +10% Attack Speed, and a +15% Attack Speed buff as well as a -15% Attack Speed debuff.
One final thing about Attack Speed: You can gain Attack Speed with AGIlity-- however, it is actually 1.0029, not 1.003 like it describes in the tooltip.
The Attack Speed CAP, on the other hand, is just a number determined by your starting Attack Speed. It just states "you cannot attack more often than this regardless of what you could have if this wasn't in effect".