Port symbols on back panel

What do the different symbols on the back panel of an iMac G5 (pre-Intel) represent? I'm assuming that the first three from left to right are USB ports (the symbol that looks like a leaf) and the next two are Firewire ports (the symbol with a circle and two lines under it) In the USB three ports I have a computer keyboard, a musical keyboard, and a mouse. The next port has a Firewire 400 connection for an external hard drive and next to it there is another Firewire port, then the internet connection which has an arrow that points left and right. The last port has a square with one line on each side of it. What is this for? I want to connect a USB mic. Do I need a hub to have more USB ports? Are they prblematic in any way?