Welcome can be a verb, a noun, or an adjective. It can also be a greeting.
used as a verb
If you welcome someone, you greet them in a friendly way when they arrive at the place where you are.
He went to the door to welcome his visitor.
used as a noun
If you want to describe the way in which someone is welcomed to a place, you can use welcome as a noun. For example, you can say that someone is given a warm welcome.
He was given a warm welcome by the President himself.
We always get a friendly welcome from the hotel staff.
You can say ‘you’re welcome' as a response when someone thanks you.
‘Thanks for the coffee.’ – ‘You’re welcome.'
You can say that someone is welcome to do something or is welcome to something, meaning that you are happy for them to do it or have it if they want.
She is welcome to stay with us while she finds a place to live.
We don't have a bath, only a shower, but you're welcome to it.
In different contexts, and with different intonation, you can say that someone is welcome to something to mean that they can have it because you do not want it and are happy to get rid of it.
If he wants my job, he's welcome to it!
used as a greeting
When someone arrives at the place where you are, you can greet them by saying ‘Welcome’ to them.
Welcome to Beijing.
Welcome home, Marta.