dont understand tires sizes. Help?

[iamjdm]

I dont understand off sets and stuff.
I see all these numbers being tossed around but i dont get them.
I understand 205/45/17. But i dont get like 7.5" +45.

Could anyone please help?
I'm new and just trying to learn so don't get on me about not knowing lol

 

[lilpoindexter]

google is your friend, son

http://www.nationaltire.com/basics/default.asp

https://www.rsracing.com/tech-wheel.html

http://www.tirerack.com/wheels/tech/techpage.jsp?techid=101

 

[HKPRO5]

The 7.5" is the width and the +45 is the offset. Those links should tell you everything.