El Pretérito y el imperfecto 

Why are there two simple past tenses in Spanish, the Preterite and the Imperfect?

The reason is because there are different ways you can envision or conceive events and descriptions in the past. You can conceive or present them

  1. as something dated (with a time limit, which can either be the beginning, the end or beginning and end) and clearly completed or 
  2. as something timeless (without caring about when something ended or began) and appearently ongoing(even though this is not the case because we are talking about something that happend in he past).
In the first case you use the Preterite and in the second the Imperfect.
 
Preterite: Imperfect:
Jugué mucho con mis hermanos cuando era niño. (I played a lot with my brothers when I was a child)

(The use of the preterite implies that I am not playing with my brothers anymore.)

Jugaba mucho con mis hermanos cuando era niño. (I used to play a lot with my brothers when I was a child.)

(The use of the imperfect doesn't tell anything about whether or not I still play with my brothers. All it says is that I used to do it.)

Su nombre fue Garzón.(His name was Garzón.)

(The use of the preterite implies somehow that his name is not Garzón anymore.)

Su nombre era Garzón.(His name was Garzón.)

(The use of the imperfect just states that his name was Garzón, and we can assume that it still is.)

But don't worry too much about this question. Spanish speakers will understand you even if you use only the preterite tense(assuming that you are using the correct verb forms). Later, when you get a better feeling for the language and when you have had enough exposure to Spanish, you will be able to decide when it is better to use the preterite and when the imperfect.

In the sections How to use the Preterite and how to use the Imperfect we will give you some rules of thumb for when to use both tenses