I have learned that there are white holes and black holes in the universe, but I am confused as to what a wormhole is. Is it a tunnel that connects black and white holes?
A wormhole is, in theory, a 'tunnel' which would be a shortcut to another point in spacetime. There's a Wikipedia article on the subject:
A wormhole, also known as an Einstein–Rosen bridge, is a hypothetical topological feature of spacetime that would be, fundamentally, a "shortcut" through spacetime. For a simple visual explanation of a wormhole, consider spacetime visualized as a two-dimensional (2D) surface. If this surface is folded along a third dimension, it allows one to picture a wormhole "bridge".
If you're the physicsy type, this might make some sense to you:
If a Minkowski spacetime contains a compact region Ω, and if the topology of Ω is of the form
Ω ~ R x Σ
where Σ is a three-manifold of the nontrivial topology, whose boundary has topology of the form
∂Σ ~ S2,
and if, furthermore, the hypersurfaces Σ are all spacelike, then the region Ω contains a quasipermanent intra-universe wormhole.
If this makes absolutely no sense to you, then you are just like me. Congratulations! For those that don't speak Greek math (such as me), here's an English explanation from space.com:
A wormhole is a theoretical passage through space-time that could create shortcuts for long journeys across the universe. Wormholes are predicted by the theory of general relativity. But be wary: wormholes bring with them the dangers of sudden collapse, high radiation and dangerous contact with exotic matter.
And here's a pretty picture of the effect:
A model of 'folded' space-time illustrates how a wormhole bridge might form with at least two mouths that are connected to a single throat or tube.
In plain English, it's a shortcut through spacetime, and the primary conceivable way time travel could happen.
A wormhole is a theorised phenomenon that would, if they exist, connect two different parts of space-time. A common analogy for the wormhole is to represent space-time as a two-dimensional sheet. If the two-dimensional sheet is bended over so that two points touch each other, than the point of contact becomes a wormhole. This has been suggested as a way to effectively travel faster than the speed of light (without actually travelling faster than the speed of light). As with most things, there are different theories about how a wormhole could exist and what its properties would be.