The 205 is a millimeter size and is a measurement taken across the tread width
The 65r or 70r is also in millimeters and is the size from the tire bead or wheel rim to the ground ( the height of the tire if you will ), The " r " stands for radial
The last number is the diameter of the tire, in your case 15 inches.
So, yes you can swap tires, and there is only a 5 mm difference which will not be noticeable to the naked eye, nor will it cause any clearance problems. If you are changing sizes however, you should change them in pairs or sets of 4, not with just one unless it is temporary, in case there are any handling issues.
You did not say what type of car you have so I can't tell you that.
Here is a site for tire information if you want to look up sizes or prices for your car.
You can replace a tire that is p205/65r15 with a tire that is p205/70r15. You need to understand that one the tire you plan on replacing is smaller than the one that you are replacing it with. This difference might not fit your car, or look the way you want it to. I'm not sure what kind of car you are putting it on, but just be careful and be sure that you replace all four tires with the same size or you will be in for a bumpy ride!