A Line Amp is designed to be installed downstream of a repeater in a DAS (Distributed Antenna System) where signal starts to drop off towards the extremities of the area thus boosting the signal back up to those weak antennas. It has both UL and DL gain, but only DL output power. A signal booster on the other hand is designed to be hardwired to the device it needs to boost or amplify signal to. It is installed between the external antenna and the device boosting both UL and DL gain.
It is important to note that line amps should not be liberally incorporated into a DAS at the design stage but should only be added if the link budget did not quiet produce the desired results after deployment, or a particular area in the DAS needs a small boost in power and a higher powered repeater or additional repeaters would be too costly to achieve this. It is always best to spec an appropriate sized repeater for the area you need to cover from inception.