A turn indicator has a gyro with an axis aligned with the longitudinal axis of the aircraft(from nose to tail). This means that it can only sense yaw.
A turn co-ordinator has the gyro axis tilted 'up' by 45 degrees, so it will sense both yaw and roll.
The indications are slightly different. The turn indicator is a needle whereas the turn co-ordinator is represented by a 'little aeroplane' viewed from the back.