You'd better read some books because your questions is very general and it could be a lot of answers here and all answers will be right but confuse you because of complexity of a subject. Everything depends on the particular goals. You could download any datasheet of an operational amplifier and to see how many parameters it has. I'm sure you don't know what they mean. The first task to understand them... hm.. you see? I've started to confuse you because of your questions. Parameters such as Vgs, Vds,Vov or Vds(sat), gm, size, and so on, is one level of abstraction. They relate to the transistors, but gain, speed, impedance, slew rate, concerned with the circuit design that is higher level, such as filters, amplifiers and etc. These parameters can be determined by a group of the transistors or even blocks. In this forum a huge amount of speculations about "I'm beginner. What book can you advice?" and you will find these topics without any problems. To me, the best book is A.Sedra "Microelectronic Circuits". Good luck