I am looking into buying a rifle. An friend of mine is a fan of the Remington 700, 7 mm.
He has been hunting for about 20 years. My question is. What is the difference when talking about cartridges and chambers when it applies to 7mm magnum, .243 Winchester, 30-06, .270 Winchester,
etc.....

Your best bet would be to look at a ballistics chart. I believe remington has a good one on their website. Trajectory, energy, and velocity are the three most important things.

Any one of the cartridges you mentioned would be fine. Most people would consider 7mm to be overkill for whitetail. .30-06 is also very powerful. .270 and .243 would offer a lot less kick. It depends on what you are looking for. I personally like .30-06 or .270, but my dad hunts deer with a .243.

By the way, Remington 700 is an excellent choice, and should offer a gun in each of those calibers. It is my favorite bolt action.

a 7mm-mag is overkill for whitetail deer. Nothing wrong with it, but unless you are going to hunt something bigger in the future, there isn't a reason for you to go that route as it may be a little much for a new hunter. I would suggest 7mm-08, 308, 270, 30-30 to name a few. all of these have less recoil, will easily take a deer or pretty much anything else in N. America. 30.06 is a fine choice too, but again maybe a little overkill for someone new. I hunt with a 30.06 and a 7mm-08. Most of time 7mm-08 due to the fact it has less recoil, lighter weight gun and devistating on deer.

If I were new to guns the first thing I would do is purchase the book Cartridges of the World. Start reading there and then go shoot some rifles of different calibers to see what you like. Also, ask around in the Guns or Firearms forum they will be a lot of help. Not that it's a bad question but it's more appropriate there. Actually if a Mod could move the post to there you'd find better results.
Anywho, start with that book. It's a little pricey $35 I think but well worth every penny.

All of those calibers are good i shoot a 7mm, .270, 6.5 x .284, .280, and .243. But if i was going to buy a new rifle I would talk a real hard look at the short mags. The short mags will give you better groups and they seem to short a little flatter. This is one of the biggest reason that i like the 6.5 x .284. The short fat case is by far the way to go. I know that you will get alot of good info on here and i dont disagree with any of it this is just my 2 cents. Check your short mags out i dont think you will go wrong with one of them.

It would be hard to beat the old reliable .270. Good selection of cartridges at good prices, good balistics and they will take out about anything in North America (and elsewhere as well). The great Jack O'Connor's favorite round.

Hammer meet nail. .30-'06 and .270 are old reliables that are incredibly versatile. Throw the .308 in that category as well.

.243 is great for light recoil, but isn't the best choice for game that's bigger than deer. (Use the heavier bullet weights on deer; the lighter ones are for varmints).

A 7mm mag is a bit heavy but will work. Recoil will be more and magnum shells tend to be more expensive.

IN MY OPINION ...... i am a fan of the 7mm Mag.
good all around gun for any animal in North america.

(then again, even a little .223 will work for anything, but not Recomended)

the 7mag is great in south texas for hogs and deer.. out west for Mule Deer.. elk..
just about anything.

Another one of my Favorates for White tail deer and Hogs..
.22-250 excellent gun. i killed my 1st 10+ deer and hogs w/ one and ive never had one take a step.
sure, it seems small.. but its GREAT FOR TEXAS

I recently purchased a Remington 700 CDl in 270. I like the fact that you can buy ammo wherever you go and there are tons of different bullet offerings. I also have a 30-06 but wanted something that is a little easier on my shoulders. Either way no matter which of the above mentioned calibers that you get you will be happy. Good luck and happy hunting.