Is a two year contract actually two years?

I just recently got a new job that will allow me to splurge a little bit and finally upgrade my phone. I hope to switch from Straight talk over to AT&T so i don't have to continue paying $300+ when I need a new phone. The iphone 6 is about $100 with AT&T s two year contract, and then I'm eligible for an upgrade. Will I be held in that two year contract or would I be able to upgrade earlier to a more recent version? How much does it generally cost to upgrade early?

No they call it 2 years because its actually 8 years.

The deals vary. But many companies allow you to upgrade up to 6 months early but there are significant disadvantages to doing it. The company has you over a barrel at that stage because you're still contractually committed; they won't offer anything like their best deals and the balance of the remaining contract gets added to the new one. So your new 24 month contract becomes a 30 month contract and if you still owed, say, 6 months @ $50 a month then that $300 gets added to the new deal and spread over 30 months.

You get far better deals in the last month because the company doesn't want to lose you. The last month of the old contract often gets waived as well. Many will start contacting you to offer upgrade deals within the final 3 months at favourable rates, but still not as good as if you wait.

In short, you don't truly get anything for nothing. Even if they allow a preferential upgrade in the last few months it's all been built into the pricing of the original contract.