From the keyboard of a single PC, with Input Director you can control any other PC on your network. From your keyboard, you can use your mouse and keyboard to control the other PC, just as if you were sitting at it.
Input Director isn't quite the same thing as remote control software. With remote control software, you also see on your monitor what is on the PC you're controlling.
With Input Director, you'll have to be able to physically see the monitor of the PC you control. So it's not nearly as useful as remote control software.
Setup of Input Director can be difficult, depending on your network and PCs. You set one PC as the slave (the computer to be controlled) and another as the master (the one doing the controlling). This can be flaky. For example, with my PCs, one PC would only work as a master - it would not work at all as a slave. So be prepared to spend time with setup.
If you need to control multiple PCs, Input Director is worth the effort. After all, Input Director is free, and that's hard to pass up.