What is the difference between masculine and feminine words?

In Spanish, all nouns (object words, like cat, house, London) are either masculine or feminine. We can usually tell if nouns are feminine because they end in 'a' and that they are masculine if they end in 'o'.

For example, gato (cat) is masculine, and casa (house) is feminine.

But why does this matter?

The way we talk about nouns changes depending on their gender: instead of simply 'the', we have 'la' for feminine nouns and 'el' for masculine nouns.

Similarly, instead of 'a/an', we use 'una' for feminine nouns and 'un' for masculine nouns.

So, a sentence describing a cat and a house would look something like:

El gato está en la casa = The cat is in the house. OR Un gato está en una casa = A cat is in a house.