Buses are the mainstay of Indonesian transport (excepting Papua and Maluku). At any time of day, thousands of buses in all shapes and sizes move thousands of people throughout Indonesia. The 'leave-when-full' school of scheduling applies to almost every service, and 'full' sometimes means the aisles are occupied too. Consider the following.
Bus tickets are cheap. For long-distance buses, you can buy your ticket from a travel agent or visit the bus terminal where you may find several companies competing for your business. Book longer trips in advance, especially on air-con buses.
Often, hotels will act as agents or buy a ticket for you and will arrange for the bus to pick you up at the hotel – they sometimes charge a few thousand rupiah for this service, but it's worth it.