Learn Spanish (Latin America): Ella compra una computadora.

In Spanish, words have a gender (masculine or feminine). Because computadora is a feminine noun (which you can often guess because it ends in -a), you must use the feminine word for "a", which is una.

Do un and una mean the exact same thing?

Yes! Both mean "a" or "an" in English. The only difference is that you match una with feminine words like computadora, and un with masculine words.

Why is the verb compra instead of comprar?

Comprar is the basic dictionary form of the verb, meaning "to buy". Because we are talking about what "she" (ella) does, we have to change the ending to match. For most verbs that end in -ar, the "he/she/it" form ends in just an -a, making it compra.

Could I just say Compra una computadora without ella?

Yes, you can! In Spanish, you can often drop words like ella (she) or él (he) if it is already clear from the conversation who you are talking about. But since compra can mean "he buys", "she buys", or "you buy" (formal), keeping ella here makes it perfectly clear who is doing the buying.