Exposure and AF

Can anyone help me clearing my following doubts?

1. i have canon 550d and i know * is for AF Lock feature. But i want to ask in which mode is it used and how to make its extensive use?

2."the AE Lock button simply holds the exposure at one setting while you recompose the image. It's very useful when spot metering, but also when dealing with subjects where you want to draw your exposure from one place, while autofocusing on another. You may hold the AE Lock button down while depressing the shutter release multiple times to maintain the same exposure across multiple shots". Can someone simplify me this?

3.On the Canon T2i, “Menu” and navigate to custom function IV-9: “Shutter/AE lock button”. Your options will be:
0: AF/AE lock
1: AE lock/AF
2: AF/AF lock, no AE lock
3: AE/AF, no AE lock

Which is the best option? #2?



Regards
Manasi C