Algunos/as; unos/as are very similar, but algunos/as is often more specific than unos/as.
1). In the singular, usually means some or any.
¿Has visto alguna mejora en sus estudios? - Have you seen some/any improvement in their studies?
2). Can be used as a pronoun.
Algunos de mis amigos son médicos - Some of my friends are doctors.
3). In the plural, algunos/as usually means the same thing as unos/unas.
Guardaré algunos/unos libros. - I ´ll keep some/a few books.
1). Often the plural form of the indefinite article can be translated as "a few" or "approximately."
2). Other times unos/unas may not be translated at all.
Ellos son unos chicos muy inteligentes - They're (some) very intelligent boys.