Love Hotels

Love hotel is a bit of a euphemism; a more accurate term would be sex hotel. They can be found in and near red light districts, but most are not in those areas. Many of them are often clustered around highway interchanges or main train stations out of the city and back to the suburbs. The entrance is usually quite discreet, and the exit is separated from the entrance (to avoid running into someone one might know). Basically you can rent a room by the night (listed as "Stay" or 宿泊 shukuhaku on the rate card, usually ¥6000-10000), a couple of hours ("Rest" or 休憩 kyūkei, around ¥3000), or off hours ("No Time Service") which are usually weekday afternoons. Beware of service charges, peak hour surcharges and taxes, which can push your bill up by 25%. Some will accept single guests, but most will not allow same sex couples or obviously underaged guests.
They are generally clean, safe, and very private. Some have exotic themes e.g, aquatics, sports, or Hello Kitty. As a traveller, rather than a a typical client, you (usually) cannot check in, drop your bags, and go out exploring. Once you leave, that is it, so they are not as convenient as proper hotels. "Stay" rates also tend to start only after 10 PM, and overstaying may incur hefty additional "Rest" charges. Many rooms have simple food and drinks in a refrigerator, and often have somewhat high charges. Before entering a love hotel, it would be wise to take some food and drinks with you. The rooms often feature amenities such as jacuzzis, wild theme decoration, costumes, karaoke machines, vibrating beds, sex-toy vending machines, and in some cases, video games. Most often, all toiletries (including condoms) are included. Sometimes the rooms have a book that acts as a log, where people record their tales and adventures for posterity. Popular love hotels may be entirely booked up in the cities on weekends.
Why are they everywhere? Consider the housing shortage that plagued post-war Japan for years, and the way people still live in extended families. If you are 28 years old and still live at home, do you really want to bring your mate back to your folks' house? Or, if you are a married couple in a 40 square meter apartment with two grade school children, do you really want to get down to it at home? Thus, the love hotel. They can be seedy, but mainly they are just practical and fulfill a social need.
One word of caution: There has been an increase in hidden cameras being planted in public and private spaces, including love hotels, either by other guests or even occasionally the hotel management. Videos of these supposed tousatsu (hidden camera) are popular in adult video stores, although many such videos are actually staged.