Both ends of the SCSI bus must be terminated. If the adapter is at the end of the bus, it usually terminates its end automatically, but this can be overridden. The other end of the SCSI bus is terminated either by the last drive or a separate terminator device at the end of the SCSI ribbon cable in Low-voltage differential (LVD) setups, or by a jumper on the last drive in single-ended setups. In some servers, the backplane has termination built-in.
In some single-ended SCSI setups, the terminator must have power in order to terminate the bus. One of the devices on the bus usually provides termination power to the bus, generally the host adapter. However, I've seen some host adapters that don't provide it, and therefore another device has to (generally the last drive on the bus). Some drives have a jumper that allows you to select whether they provide termination power or not. If the host adapter doesn't provide it, and the drive is jumpered to not provide it, then the bus won't be terminated even though the terminator is present.
Every device on the bus, including the host adapter, must have a unique ID. If you have an older single-ended bus where there are some narrow/8-bit devices and some wide/16-bit devices on the same bus then know that narrow devices do not see the high IDs (8-15). Every device needs to use an ID between 0-7. Buses with only 16-bit devices have ID's 0-15 available. All LVD devices are 16-bit.