Radio frequency above 1GHz is called microwave frequency.
Microwaves have some special properties along with that of any electromagnetic waves. Like
1) They have very high directivity. Due to this they require very small antenna.
2) Free Space Path Losses at certain microwave frequencies are very less.
3) They can penetrate ionosphere and hence are used for satellite communication. And many more.....
Basically microwaves are used for point to point communication due to their high directivity property.
go to google.com and type in microwave communication. There's lots of info on the net to sift through.
If you really want to get indepth, you should buy a book on radio communication. This will give you a base to learn microwave stuff. Its impossible to learn microwave communication without knowing lots about radio