as a noun, 'welcome' refers to the act or an instance of greeting someone.
The warm welcome we received was unforgettable.
The event began with a welcome from the principal.
The celebrity received a hero's welcome upon returning to his hometown.
'welcome' as a noun can also refer to the feeling or reaction induced by someone's arrival, as in 'They gave him a cold welcome.'
as an adjective, 'welcome' describes something that is pleasantly received or invited.
Your feedback is always welcome.
A welcome change in the weather is expected tomorrow.
The news was a welcome relief for the anxious family.
'you're welcome' is a common response to 'thank you,' but in this context, 'welcome' is still an adjective, indicating that the gratitude (or whatever was given) was gladly received.
as a verb, 'welcome' means to greet (someone) in a warm or friendly manner upon their arrival.
'welcome' can be used transitively (with a direct object) or intransitively (without a direct object).
We welcomed the guests at the entrance.
The community welcomes newcomers with open arms.
The mayor will welcome the delegates tomorrow.
'welcome' can also mean to be pleased to accept something, as in 'I would welcome some assistance.'
as an interjection, 'welcome' is an exclamation used to greet someone or express pleasure at their arrival.
'Welcome! It's so good to see you!'
'Welcome to New York!'
'You made it! Welcome!'