Basically, it breaks down like this:
Subscriber - Obviously, paying a subscription for the game.
Preferred - Anyone who previously subscribed OR anyone who spends at least $4.99 USD for Cartel Coins, the Digital Upgrade pack, or the Physical Security Key. Once you have a Preferred account, it stays that way unless you choose to Subscribe again. If you do, you'll drop back to Preferred status when the subscription runs out.
Free - People who haven't given EA/Bioware a dime.
Really, it's worth it to at least buy one set of Cartel Coins to get up to Preferred status.