Copying free themes and plugins:
These are all under the same license as WordPress (GPL) or a very similar license. Basically, you can copy them, make changes to them, use them on multiple sites, send a copy to a friend, etc.
You can not sell a theme or plugin under the GPL (general public license) whether it has been modified or not.
Copying premium themes and plugins:
If you buy a premium theme or plugin, the license will depend on where you purchased it. Themes sold on wordpress.com generally are all under the GPL license. If you buy a theme on another site, ThemeForest, MojoMarketplace, .etc, then the license is likely different. So, make sure you check before copying it or sharing.
Any premium theme or plugin, however, may be designed so that only one site can access the code per purchase (probably with an API connection).