Curses are most commonly placed unintentionally when someone has very strong negative feelings about someone. The energy you direct at this person begins to draw in its own energy, and it can grow and grow and be hard for this person to get out of. Curses aren't like Hollywood magick, and they mainly work psychologically on a person. They can be intentionally placed on someone, but this takes a lot more work than shown in the movies to get the curse to have any real effect.
Places can be cursed; sometimes there are just places of very negative energy, just like there are places of very positive/good/happy energy. Items are probably the most common thing to be cursed, like paintings and chairs (Napoleon's chair for example), whatever.
As for breaking curses, try banishing spells and lucky amulets, as well as setting up many protective barriers.