There are two past tenses of ser... the preterite and the imperfect.
This site will explain the differences.
Just in case that English explanation is not too clear, the difference between the preterit and the imperfect is one of my favorite things to teach!
The Preterit narrates a completed action or event, similar to a snap shot. It has a beginning and an end. The English meaning will be something like: I read, I DID read.
The Imperfect describes a situaton, similar to a moving picture. We know it had a beginning, and we assume it will have an ending, but we are focusing on the middle part. The English meaning will be something like: I read (same meaning as the preterit and this is what causes the confusion often), I WAS readING, I USED TO read.
The preterit has some often used verbs with special meanings. "querer" will be "intended" or in the negative, "refused." "estar" will be a synonym of "llegar" = "arrived" "poder" will be "managed, could" "tener" is "got" in the sense almost of "grabbed" I'm sure at this point you are not concerned with that.
It takes time to get a "feeling" for the difference between the two. When you do some exercises, be sure to check with us if you aren't certain.
P.S. I forgot to mention that both "ir" and "ser" are identical in the preterit. You can tell it's "ir" because of an "a" when you state "to where" someone went.
Examples: El fue estudiante. (ser) He was a student. El fue a la biblioteca. (ir) He went to the library.
Wow, GuruBlue, what doesn't Wikipedia have these days?!