Resources can be stolen from another player by any of your resource transports. To steal resources you must move the resource transport within range of the other players Central Processor or Command Relay, and the enemy structure must be deactivated. A green circle will appear around the enemy Central Processor or Command Relay indicating the range. Then click the "Steal" button on the information panel.
The amount of a resource that can be stolen from any Central Processor or Command Relay is equal to the total capacity for the resource, minus 20%. This means that only 80% of the capacity of a given resource can be stolen.