Particle - が - Subject - subject-marking particle
The particle が is used to mark a subject in a sentence. The particle is very similar to the particle は (Topic marker) which makes it troublesome for people learning Japanese. The difference is usually in the emphasis: 猫が歩きます (The cat is walking) vs 猫は歩きます (The cat is walking). The particle is always used instead of は to mark the answer when answering questions.
Note that the subject with this particle needs to be well-defined. This means that you cannot use the particle や in the subject.
Oh, there's a butterfly!
Today I have a lot of things to do.
I like cats
This movie is worth seeing again.
It can't be helped
It was raining today
I like cheesecake.
I want to drink tea or coffee
The hospital is nearby
Here's some water