To begin, I was ordained as a monk in Thailand for two months at the end of 2007. It's not typical for foreigners to become a monk, and typically wouldn't make sense for someone to, but I'll explain the background and what is involved.
My own background: I ordained at Wat Pho in Bangkok (the "reclining Buddha" temple). This was related to a prior study of Buddhism, and even more related to being a member of a Thai Buddhist family. There are no formal limitations on who can or cannot ordain but without personal knowledge of a candidate monks in a temple would typically not accept ordaining anyone. Typically it's like being a member of a Christian church, just in a different form; it's a role related to that personal involvement as a "church member."
Thais ordaining as temporary monks: it is considered a rite of passage for young Thai males to become temporary monks at some point in their lives. Often a death in the family is a trigger for this, since merit (positive karma) is generated through this activity, and some of that can pass on to family members, even those who have already died.
Why was there ever a temporary monk role? There was some allowance in the early Buddhist tradition for men to ordain during the rainy season, and then return to agricultural work during the rest of the year. My understanding is that this was more related to the spiritual role than to them being supported by the community with food offerings for those three months, but a social-support purpose may have overlapped. The rainy season paradigm is no longer used, with modern temporary ordinations typically for two weeks only, often also in observance of a significant event, like the birthday of the Thai King.
Can women be monks? The short answer is no, with a few qualifications. Originally women were able to ordain as a form of monks in the Buddhist tradition, per early teachings. One "renegade" Thai Buddhist group in Australia has ordained at least one woman, per my understanding, but this action was later rejected by central Thai Buddhist leaders. Women can serve in a similar role in Thailand today, as nuns, just without taking the same types of vows and without serving a completely identical role.
Do monks need to be Buddhists? An odd question, right? During my stay as a monk people did ask if I was a Christian, and without Buddhism being as faith-based as Christianity, more activity / ritual / role based, it doesn't make as much difference. Obviously it makes sense that anyone ordaining would accept all related Buddhist teachings as completely valid, to be Buddhist.
How many rules (restrictions) are there for monks, and how would one learn them? There are more than 200, and versions of the rules are available online, some of which require interpretation. Many are basic, like monks are required to only wear robes. Others: monks cannot eat after noon (technically sit down to a mid-day meal after noon, but roughly the same); monks cannot sleep in typical "high" beds, they cannot drink alcohol, cannot touch women in any way, not even to the extent of a woman handing something directly to a monk, and cannot own or handle money. Some seem a bit obsolete but in general every rule is followed very strictly and very literally, with some exceptions. Typically new monks are given classes or direct instruction by other more senior monks.
What is the ordination process like? The hardest part is learning the vows, substantial chanting in Pali language. Beyond that there is a ritual shaving of a new monk's head (Thai monks shave their heads monthly, not frequently like Zen monks), and other ritual steps common to other ceremonies. One difficult process is that all the rules apply as soon as you are ordained, with additional expectations related to observing the demeanor of a monk, so all that can be a bit of a shock.
What are monk's duties? It varies by temple, but in general monks go on alms rounds (receive morning offerings), attend daily religious observations, and take part in ongoing ceremonies, memorials or weddings, etc. Other roles might involve helping set up for ceremonies, cleaning up living areas, or helping care for elderly monks. A temporary monk might take up meditation practices, or might not have a lot to do, depending on guidance and oversight.
Would it make sense for someone to ordain for a meditation retreat, or just to have a unique religious experience? No, not really. There are meditation retreat centers for that. It's most valid for active participation as a member of the Thai Buddhist religion, based on religious beliefs, and grounded in the context of ongoing observation of a lay-person's religious role. There is a spiritual-experience vacation agency that ordains foreigners as novices (samananes, the role typically reserved for children), but in general going on meditation retreat as a lay-person is a similar and more appropriate experience. For a Thai Buddhist there is a difference related to karmic (merit) effects of participating in this role but for a non-Buddhist this would seem to not be relevant.
I wrote more about this experience, personal accounts of conditions, and more on rules and restrictions in a blog post: