These are small inexpensive minibuses that the locals mostly travel on. So using them will ensure you get to meet the real islanders; the one's who most likely work outside the tourist industry. Bemos start very early in the morning, then start getting sparser as you get closer to 5pm; remember this if you are planning a day out. Bemos do not have air conditioning, are often busy and can therefore get really steamy! So be warned.
Bemos can get you virtually everywhere around Bali & Lombok, but the more remote areas will have infrequent services. Very remote places such as Amed may have no regular bemo service. So, if you want to get there, you would need to take a bemo to Culik and then negotiate a ride with a local or walk the last 5km. Bemos can be flagged down anywhere you see them (they will probably double honk their horn as they approach tourists & locals alike while they are walking along the roadside). If you are carrying luggage / rucksacks, you should not be charged extra for this as long as it can sit on your lap. As to how much to pay, when the driver tells you the price, expect the worse, look shocked, offer a third of what they said (ready to pay half the amount) and then ask a local on the same bus how much extra you paid then you should (for the next time)! But don't end up with your principles intact, still on the roadside, watching the tail end of a bemo, driven by an offended driver, disappearing in the distance! If you have the nerve, you can try just getting on, waiting for the designated fare collector to come around and watch closely at what the locals pay.