What is a one masted vessel with a mainsail and jib called?

Answer 1

The most common single masted sailing vessel with a mainsail and one jib is the sloop.

A cutter has one mast with one mainsail but two jibs.

A catboat has one mast and a huge mainsail but (usually) no jib. The mast is located very much forward on the vessel.

There are other sailboats that have just one mast and a mainsail such as the Sunfish or a Dhow, both of which have a lateen rig (one mast, one mainsail.)

Answer 2

That vessel is said to be 'sloop-rigged', even if that jibs'l is a spinnaker with whisker pole(s).