Ulpan courses are available for everyone. There are some courses which are only available to new immigrants (olim), but most courses integrate olim and tourists. Different ulpan have different intensity levels.
Before registering for an ulpan, consider your language level. There are six ulpan courses: Aleph, Bet, Gimel, Dalet, Hey and Vav. (These are the first six letters of the Hebrew alphabet.) The first three levels are beginner courses, and the last three levels are advanced courses. A single course takes about 6-8 weeks.
Aleph introduces students to the alphabet, basic greetings, and important questions like “Where is the WC?”. The next levels, Bet and Gimel, build vocabulary and introduce you to basic tenses. An introductory six-month ulpan will usually include the first three courses.
By the end of an Aleph-Bet-Gimel course, you should know enough Hebrew to get by on the street, write a quick note, and carry a decent conversation - with a few occasional gesticulations to get your point across.
Dalet, Hey, and Vav courses are for individuals who want to gain the maximum proficiency in Hebrew. These last three courses focus on comprehension and written skills. Students in Dalet courses listen to radio broadcasts and read newspapers, Hey courses focus on the nuances of grammar, and Vav courses teach writing and the basics of Hebrew literary analysis.
If you have some background in Hebrew, you may be able to skip the introductory courses. If you're unsure, ask your prospective ulpan about Hebrew language proficiency tests.
Other types of ulpan
Some agricultural co-operations (kibbutzim) offer ulpan courses as part of cultural enrichment for new immigrants. Hebrew University in Jerusalem offers ulpan courses for university credit. Religious ulpan courses not only teach Hebrew, but they give a good background in the Torah and religious observation. There are also ulpan courses for youth, tourists-only, and women. A list of ulpan courses can be found on the Jewish Agency's website.