The Q ships were those that pretended to be merchant ships, with hidden guns, and not flying the Naval ensign(until ready to fire). They didn't travel with convoys, they travelled alone, since a U boat wouldn't fear them, and would not waste a torpedo on them.

There were also armed merchant cruisers, merchant ships that were refitted, sailed with convoys etc.

The Germans also took merchant ships and made "Q" like ships, that could pass as merchants but who were armed liked cruisers. The Kormoran was the most famous of these, managing to sink an Australian light cruiser. They even had scout planes, and torpedo tubes.