Router on a stick (or One Armed Router) is a common name for a configuration used for routing between VLANs on a single Ethernet (including Fast/Gig/10Gig) interface. This configuration uses sub-interfaces on the separate VLANs and an 802.1q or ISL trunk to perform the routing. There are a few reasons you would use this configuration, and several not to. Probably the most common reason is cost. Most branches will have a router for WAN or internet access, using the same router for routing between VLANs saves the cost of a Layer 3 switch. You may also wish to make use of some of the more advanced features on a router, such as firewalling. Another thing that cuts both ways is that you gain some simplicity by minimizing the number of routing devices, but you add complexity to the configuration of the router. The main drawback is performance. You are pushing all your traffic between two VLANs through a single interface, which could become a network bottleneck. You have to determine the potential impact to your network. The topology and network usage will largely determine how big an impact this will have. If you have all your PCs on one VLAN, and a couple servers that you are doing large file transfers with on another, and your printers on their own VLAN, you are probably going to see some serious issues. Conversely, if you have a single data VLAN, a wireless VLAN, and a voice VLAN, with the majority of the traffic from each going back to a central site over a WAN connection, this may be a very legitimate production use of router on a stick.
Configuring Router on a Stick