Lorentzian wormholes are essentially short-cuts through space and time. They are mainly studied by experts in Einstein gravity, and if they exist in real life would be more-or-less similar to the wormhole on Star Trek: Deep Space 9. (But remember, the show is just entertainment, so don't try to extract detailed physics from DS9; at best it will give you a vague general idea of what is going on.)
The good news about Lorentzian wormholes is that, after about ten years of hard work, we cannot prove that they do not exist. The bad news is that they are very strange objects: If they exist at all they need large amounts of negative mass to hold them open and stop them from collapsing. (Negative mass is not anti-matter, it's a region where the energy of the universe is less than that of ordinary vacuum---definitely weird stuff.) We can get small amounts of negative energy in the laboratory (the Casimir effect), but getting the large amounts needed to hold a decent size Lorentzian wormhole open looks to be hopeless with current technologies. (And there may be deep issues of principle preventing us from collecting a lot of negative energy in one place.)
If Lorentzian wormholes do exist, then it seems classically to be relatively easy to turn them into time machines. This embarrassing feature has led Stephen Hawking to promulgate his Chronology Protection Conjecture. According to this conjecture, quantum effects will conspire to effectively prevent time travel even when it looks like classical physics might allow time travel to occur.
Euclidean wormholes are even stranger: they live in "imaginary time" and are intrinsically virtual quantum mechanical processes. These Euclidean wormholes are of interest mainly to the particle physicists (quantum field theorists). You cannot give them a nice classical interpretation in terms of a well-behaved classical gravitational field, and unfortunately have to know a lot of quantum physics to appreciate even their basic properties.
A good popular level description of Lorentzian wormholes can be found in the book Black Holes and Time Warps: Einstein's Outrageous Legacy by Kip. S. Thorne (Norton, New York, 1994).
The BBC has a documentary in the Horizon series: The Time Lords, Judith Bunting, December 2, 1996.
If you know some differential geometry, some general relativity and some quantum field theory (not for the faint of heart), you might want to take a look at Lorentzian Wormholes: from Einstein to Hawking by Matt Visser (AIP Press, New York, 1995).