When your membership expires, your profile will be kept on the system and will still be visible to other members, but you'll be limited as to what you can do on the site. You'll still be able to read messages from other members, but you'll have to subscribe in order to reply to them. Compare membership types.
If you think you might want to come back at a later date but want to hide your profile from other members, visit Your Account page and under Privacy tick Hide my profile from everyone.
If you don't log in for an extended period of time, your profile will eventually be removed and your account closed.