Squire
A squire is an attendant of a knight who may act as their shield-bearer and assist them in various other ways. Traditionally, they are ranked below the knight but higher than a gentleman. There is no gaurantee that they will ever become a knight themselves.
It came to also mean an English country gentleman. A related term "esquire" still exists in England to this day.
Squires in the SCA
This is a hard topic to discuss, as squiredom can mean many things to many different people in the SCA. In my view, being a squire does NOT mean that you are any better than any other member of the society. Different knights choose squires for different reasons, and expect different things from their squires. It is something that has to be discussed when the opportunity arises. I believe squiredom is a relationship where a knight believes that he has something to offer his squire, and is mutually beneficial to both parties. At the bare minimum, this would normally mean that the knight will help the squire in their combat training, but may also aim to help the squire better understand and practise chivalric behaviour.
Squires can be recognised by a red belt that often has their knight's device painted on the end. This shows that you have fealty and a duty to your knight.
As I said, there are many different views with regards to squiredom and my views may totally go against those of another person.