On Google+ I was filling out my profile when I came across the "Contributor" field; before filling it out, I thought to ask. I have been on Stack Exchange for a while; should I be counted as a "contributor" or "just a user"?
SE has no rules as to how you represent your participation in the network outside the network.
However, you said in a comment:
I may use that profile in professional network sometimes[.]
You should be concerned about how other people are going to perceive the terms you use to describe your participation against what is publicly visible in your profile, and on the SE network. If someone in a professional setting looks at your profile, is your activity on the network going to justify the terms you use? People have different standards as to what constitutes "contributing" to a project or a community. If your activity on the network does not rise up to their standard, they are going to perceive your description as a misrepresentation. Sticking to facts (e.g. "I have posted over 200 answers.") would be safer than using terms like "contributor".
If the facts you can highlight seem underwhelming to you, then this suggests that maybe your activity on the SE network is not noteworthy.
As Martijn said, you can count yourself as anything you want on Google+. (did you think your G+ profile was going to get audited or something?) But besides, I don't even think there's a difference. People on Stack Exchange pretty much interchangeably use the words "user", "contributor", and "member" to refer to anyone who has posted, voted, edited, or sometimes even just created an account.
I will draw a distinction between Ask Ubuntu the site, Stack Exchange the platform and Stack Exchange the company.
- You are a contributor to Ask Ubuntu the site.
- You are a user of Stack Exchange the platform (in one of two ways: a user can be one who deploys the platform for their need, and can be one who participates in a such a setup). A contributor here would be a member of the SE development team (and other such teams).
- You are the product of Stack Exchange the company (the way users are the product of advertising services).