I see. IMHO, I think you got confused with the terminology of understeer and oversteer.
You say: "cause u can only drift on a straight line". Technically speaking, you can't drift in a straight line. When you drift, the path your vehicle follows will always be in an arch. The faster you go, the bigger the arch. Vice versa. If the corner tightens, there are various techniques you can apply, I won't go into details but generally speaking, you need to put in more countersteer.
Now, to define understeer, it means:
1) Your front tyres are having a bigger slip angle compare to the rear tyres - When you're drifting, your rear tyres are constantly spinning and having a bigger slip angle than the front tyres, which is what gives you the 'sideways' feeling seeing a car drifting. Although the front tyres do slip at a minimal angle (technically speaking you won't get 100% grip up front due to caster angle), they do not go into 'lost traction' mode like the rears as that will probably send the car running wide and all hell break loose. If you 'understeer' in a drift, it means the rear wheels are no longer in a slip-angle greater than the front, which gives you the 'straighten' feeling (ie not 'sideways' anymore)
2) Your car is turning less than your steering input - When you're drifting, your car is always turning more than your steering input. Why? That's because you're in opposite lock. Think about it. If you understeer, you turn into the corner fully but your car is only responding little. If you oversteer, you don't have to turn the steering and the car is already rotating towards corner. In fact, you have to turn much much more lesser, till the extent that you need to turn the steering away from the corner (ie counter-steering or opposite lock) in order for the car not to spin. If you 'understeer' in a drift, it means you're no longer in countersteer.
Oversteer is what makes the car slide or drift. Drifting is about holding the car in oversteer. How well you control and hold the vehicle in 'oversteer' mode like it's still in grip mode is what makes the world class top drifters today, where they are doing ridiculous speed while holding the drift angle at a fringe of 'over'-oversteer ie SPIN.
There are some usefull information to aid your understanding better. Do some read-up:
http://en.wikipedia.org/wiki/Understeer
http://en.wikipedia.org/wiki/Oversteer
...oh and if you have more time, do check it out here too:
http://www.drift.com.my