The Dropship provides 8 slots for transporting.

Marines, Firebats, and Ghosts each use 1 slot.
Vultures and Goliaths use 2 slots apiece.
Tanks use 4 slots.

Dropships must unload units one by one at a fairly slow pace. Dropships can pick up units much faster than they can unload them. Avoid unloading transports while they are under attack because of this fact. You want to pick out of the way spots or spots free of anti-air fire to unload your units rather than unloading them in a hot zone.

The easiest way to load a transport is to select units and right click on the Dropship. The Dropship will move to intercept them.

A very useful strategy is the Marine drop. Place 8 Marines in the Dropship and drop the units in the middle of unprotected mining operations. Make sure you defend against this strategy yourself by placing anti-air defenses around your resources.

Other useful payloads include 1 Tank and 4 Marines for drop on high ground.

Remember to use way points to guide your Dropships around obstacles for a safe path to its destination.

If your Dropships come under attack run away from the attack. If that is not possible, try to unload the Dropship as quickly as possible. You want to avoid losing full transports.

Dropships can be used to extend your viewing range of the battle allowing your units to fire further (especially Siege Tanks in siege mode)

If a Barracks trained Infantry unit with Irradiate on it is placed in a Dropship, all other Infantry units and SCVs are damaged by the Irradiate. Irradiating the Dropship itself does not hurt anyone inside however. The same is the case with Bunkers.

Cast a Defensive Matrix on Dropships then send them into enemy defenses. This gives them an additional 250 hit points. This might allow you to land troops in heavily defended areas especially islands.

Units can be unloaded while the Dropship is in motion by clicking on the unload all button and selecting the Dropship as the target.

Remember you can repair Dropships, keep them at full health.