What is the difference between ToS and terms-and-conditions?

I see in this board the two tags:

• ToS (which I understood as "The Terms of Service")
• terms-and-conditions

The same I see for on-line services. Some companies, providing on-line services, have "The Terms and Conditions" while the others "The Terms of Services"

What are the differences between these terms?

Are they synonyms?

"Terms & Conditions" and "Terms of Service" and "Terms of Use" are synonyms. I like "Terms of Use" a little better, as it in my opinion more precisely communicates that using the service implies accepting the terms. But all 3 are perfectly usable.

Keep in mind that there is a lot more to be said about "Terms of Use" than just the naming. How the end user enters into contract (explicitly clicking "I Agree" versus just using a site which has some terms somewhere), how the terms can be changed and how the end user is notified of these changes, and especially what the terms actually say...

If you're interested, the terms-and-conditions tag and the similar Quora topic have lots of good posts.

You're right about there being two tags here which are essentially identical. It happens because all users with some reputation can create tags, and we don't all 'think' in the same words. Eventually they will probably get merged.