Data directed from the host to a device is called downstream or OUT transfer. The other direction is called upstream or IN transfer. It is used to configure devices and every one is required to support a minimum set of control commands. Bulk transfers are used to request or send reliable data packets up to the full bus bandwidth. Devices like scanners or scsi adapters use this transfer type.
Interrupt transfers are similar to bulk transfers which are polled periodically.
Isochronous transfers send or receive data streams in realtime with guaranteed bus bandwidth but without any reliability.