The verb estar is an irregular verb, that means, it does not follow the pattern of all other "regular" -AR verbs.
The verb estar means "To Be" for temporary characteristics.
Think LCP (Large Cheese Pizza)
L- Location
C- Condition
P- Participle.
Those are when you use estar.
Conjugations of estar:
I am yo estoy
you are(Informal) tu estás
he/she is / you are(Formal) el/ella está // ud. está
We are nosotros estamos
You all are (spain only) - Vosotros Estáis
They are Ellos/Ellas/Uds. Estan
Sentence: I am happy today
Yo estoy feliz hoy