Elaboration of terms at BITS

There are many terms used frequently here in BITS, out of which I know only a few. Could someone please explain the following terms to me as if I'm a newbie?

What is SI?

ET? (Not exit tests)

Offshoots?

Electives? (I know these are subjects apart from your CDCs that we need to take to complete our credit requirements, but like what is the difference between the types of electives, how many of each type do we need to take, how do they matter etc.)

Summer terms and their need (from a dualite's pov)

If there is anything apart from these which are not common knowledge to a fresher, please explain those as well.

Thank you!