I have been hospitalized once. I came home one night and told my (then) husband that if he didn't take me somewhere to get help, I was going to kill myself. He took me to the ER, and they put me in an awful, tiny room with blank walls and nothing in it except a bed. They did blood tests (to see what drugs were in my system) and observed me for several hours. They decided to admit me to the psych ward for 7 days on suicide watch.
The hospital itself wasn't too bad. Honestly, I slept for the first 2 or 3 days because I was coming off a number of drugs. There were set meal times and the food was similar to school food. There was group therapy, art group, individual meetings with the doctor, medication times, visitation times, phone times, and my p doc visited me twice. There was a patio that we were allowed to go out on sometimes. They did provide scrubs to wear, but I was also allowed to wear my own clothing with some exceptions (no strings on hoodies, etc..). When I wanted to have a bath and shave my legs, I had to be supervised. They checked our rooms every two hours at night. There were 12 patients in my ward. Significantly more males than females. There were 2 doctors, an art therapist, a few nurses, a councilor, and some other staff. There were some very interesting people there.
After 7 days I was released to the care of my husband, with the understanding that I would be entering an inpatient rehab right away. He was given instructions not to leave me alone long, not to allow me to have access to medications or weapons of any kind. All of the knives in our home were put into a safe. I was home for 3 days before I entered rehab. This was 3 years ago.
Recently I came VERY close to admitting myself to the hospital again. In fact, the only reason I didn't was because it was a few days before Christmas and I didn't want to ruin Christmas for everyone (so odd how our minds work...). I told a few people about how I felt, and I was not left alone for several days. After Christmas, I had come around the corner and did not feel like I needed to be hospitalized. If there was a time when I truly felt that I couldn't keep myself safe, I would not hesitate to go back in.
Sometimes we need more help than we can get in outpatient therapy. It's ok. For me, I think the experience of being in the psych ward, being completely separated from my comfort zone, was a huge part in making me realize that this isn't what I wanted for my life. I needed the hospital to break the cycle I was in mentally.
From the research I've done, the exact laws vary from state to state, but in the US, you can be committed involuntarily for 72 hours if a doctor believes you pose a serious risk to yourself. The initial hold can only be 72 hours, after which there has to be a hearing to determine if you need to be held longer (that is if you are objecting to being held). I imagine that if your therapist thought you needed to be admitted, he/she has a doctor that they work with that would make that recommendation.
I think, however, that if your therapist were to suggest hospitalization, it's something you should seriously consider. Obviously you are the only one who really knows where you're at mentally, but sometimes I think we need an outsider to tell us when we could really use some additional help. But that's just my opinion.

I hope that helps. Good luck