Nouns, i.e. the words for people, things, places and ideas, are all either masculine (m.) or feminine (f.) in Italian. It is important to know whether a noun is masculine or feminine because there are m. and f. words for "a" and "the", and m. and f. versions of adjectives (describing words).
A useful general rule to remember is that, nearly always, nouns ending in -o are masculine and nouns ending in -a are feminine. Nouns ending in other letters can be either.
il, lo, l', la, i, gli, le
un, uno, una, un'