instrumentation amplifier

You must have forgotten about internet search engines.

Google

Are you taking a course in controls? If so, you should be doing your own homework.

If you have a job as in instrumentation person, you should know the answers to these questions already.

http://en.wikipedia.org/wiki/Instrumentation_amplifier

Please perform a basic internet search first for simple questions such as this, the information is readily available.

Yes, good start. And while you're looking debasis, consider the three types of closely-related amplifiers we use these days: difference amplifiers, instrumentation amplifiers, and differential amplifiers, sometimes called full-differential amplifiers. The latter has come back into use with modern high-performance A/D converters. Learn the differences between these types, parden the pun.

Another thing to understand: common-mode rejection. Consider this in the light of a) unbalanced input source impedances, and b) high frequency common-mode signals. Think about interfering noise signals and their types.