Spacers, what are they??

Sorry for the n00b question, but what are they and how do you know if you need to use them?

I would like to put some Cragar 345 Series Silver "D" Window 16x8, offset 13mm, Rearspacer 5 on my 5lug. I plan on doing a Readylift 2.75"F/1.75"R what I would like to know is will I have any problems like having to replace the wheel studs or putting spacers? If I do I just might have to go for some other rims.

 

http://www.roughcountry.com/images_large/jeep_1090.jpg
These are wheel spacers. You bolt them on as you would your wheels and then bolt your wheels on to them. They space your wheels out widening your stance and give you more clearance between say tires and upper control arms.

 

Is it a necessity when putting after market rims?

 

Not all rims. If you don't have the proper backspacing you are required to use spacers to clear the brake calipers or if you run fat tires so they don't rub the upper control arm (they might still rub other pieces of the body though depending on the tire size). I don't know much about what backspacing requires spacers, there are others who know a lot about this issue. There have been a lot of threads about this so worst case if you search backspacing I'm sure you'll get a lot of hits.