Are you talking about a trainer — as in the device you mount your bicycle to and ride in place?

They clamp to your rear skewer and your tire turns a flywheel that has either magnetic (less expensive) or fluid resistance.

What do they build on? They allow you to keep your legs when it is too miserable outside to ride.

And here is what they look like

EEEEK! I’m the first to answer this, so I may have interpreted your question wrong, but here goes!

A cross trainer is someone who cross trains (surprisingly enough)

ie. a cyclist who does some running or lifts weights.

They build on little used muscle groups used outside of their discipline.

EDIT: I know what you mean now! They are those horrible looking bike things you find in a Gym.
You may find an indoor trainer better for you if you are looking to improve your cycling.
This is the one that I have:

Indoor trainers or ‘turbos’ can be used to improve: Cadence; Endurance; Power; Hill climbing; etc. depending on how you use it.
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