The difference between definite articles (“determinados“) and indefinite articles (“indeterminados“) can be observed in the following two sentences:
Give me the blue pen (a particular or definite pen)
Give me a pen, please (any of a number of pens or an indefinite pen)
The difference between definite and indefinite articles is the difference between talking about a specific pen, or any pen.
-> the pen
-> a pen
DETERMINADO (= the)
INDETERMINADO (= a/ an)
Tips about articles in Spanish
Unos / unas is also used to express approximation:
Laura tiene unos 15 años
Laura is around 15 years old
The masculine plural definite and indefinite articles (los / unos) are also used to indicate a group of mixed sex. For instance, “los gatos” could refer to a group of 10 male cats or it could refer to a group of 9 female cats and one male cat.