RS422, RS485 Termination Resistors
When you send a high speed signal down a cable, the current that initially flows is dictated by the voltage applied AND the characteristic impedance of the cable. For the types of cable recommended for RS485 and RS422, the cable characteristic impedance is circa 100 ohms.

So if 1 volt is applied at the sending end an initial current of 10 mA flows and all is well until the end of the cable is reached; voltage and current expect to see a continuing impedance of 100 ohms. If it weren't 100 ohms then what does 1 volt and 10 mA make of a mismatch?

The answer is that not all the power of the 1 volt and 10 mA is used at the receiving end and some power gets reflected back up the cable towards the source. For slow speed signals this isn't a problem but for higher speed signals this reflection damages the integrity of the data edges (for up to several micro seconds in some cases) and can corrupt the data. Here's a .gif file image that shows the general idea: -



A pulse enters from the left and hits a transmission line impedance anomaly (as indicated by the vertical black line). A decent proportion of the pulse energy continues to flow from left to right but, there is also a reflection at the "anomaly" that flows back to the originating source. The reflection can cause data errors.

Branch-off points are always regarded as having very little length and so termination resistors can be ignored but, in addition, you can't attach several termination resistors because these would alter the transmission impedance as the signal passed down the cable and this would also create a mismatch and produce reflections and corrupt data.

Multi-point systems need termination resistors at both ends of the cable because data is sent bidirectionally.