Hola Skyler

Estuvo is from "estar" and this is the 3rd person singular in the preterite/ past simple:

Carlos estuvo en el bar - Carlos was at the bar

You may be also thinking of "estaba" which is also from "estar" but in the imperfect tense:

Carlos estaba en el bar - Carlos was at the bar

As you can see, the meanings are the same, but the preterite is used to express an action that we see as complete, and the imperfect for actions that are there more simple to describe what was happening. 

"Era" is the 3rd person singular of the imperfect of "ser" (the other "to be") and we'd use "era" also for a descriptive purpose or a "used to be" meaning,  more than expressing an action that we see as finished (that would be the preterite "fue"):

María era muy guapa.  - María was very pretty.

Era una tarde tranquila- It was a quiet afternoon. 

There are lots of lessons from A2 to C1 that explain differences and specific nuances of ser and estar, and also lessons that deal with the difference in using the preterite or the imperfect. 

Here are some if you would like to add some to your notebook:

Estar (preterite)

Estar (imperfect)

Ser (preterite)

Ser (imperfect) 

Ser in the imperfect vs the preterite

Estar in the imperfect vs the preterite

 

I hope they help.

Saludos