Tire Sizes....???

[ashx2]

The tires that were on my '72 when I purchased it have gone south on me. Tire sizing is not exactly my comfort zone and don't know a lot about it. The current tires I have are P275/60R15. I have a neighbor down the road that has some tires that are P265/70R15..... What will be the difference, the height of the tire? Or how does that work? I'm assuming that the 15 is the size of the wheel itself, so that should be a doable thing.....or am I off base on this? Thanks