Trichomoniasis (often abbreviated as "trich") is an STD caused by a protozoa (a single celled organism that can move itself around) infection. In an unfair twist of nature, trich is primarily a woman's disease. While men can become infected, they do not have symptoms and the infection goes away of its own course. Infected women, however, suffer from a variety of symptoms, including smelly vaginal discharge, a burning sensation when urinating, and vaginal itching. Together these symptoms are referred to as "vaginitis" which means swelling and irritation of the vagina.
If you find yourself with symptoms of vaginitis, do not automatically assume that you have Trichomoniasis. Not all types of vaginitis are STDs. Vaginitis can be caused by bacteria or fungal overgrowth infections that are not necessarily sexually transmitted.
How you have arrived at vaginitis symptoms is not terribly important if you have them. What is important is that you seek a doctor's care. Your doctor will take a sample of your vaginal fluid and send it to a lab for testing. Women who test positive for the Trichomoniasis protozoa are generally prescribed medications (Metronidazole or Tinidazole pills in one dose or multiple dose formats; note that people taking Metronidazole should not drink alcohol!) which eliminate the protozoa from the body.
As is the case for other STDs, recent sexual partners should be notified. Male sexual partners need not be tested; it is okay for them to just take the medicine.