Which iMac G3 models have Firewire "400" ports?

In a delightfully easy-to-remember manner, all iMac G3 models with 400 MHz or faster processors include two Firewire "400" ports.

Why USB? What advantages does USB have over earlier connectivity standards?

The original answer published on August 17, 1998 follows:

The primary advantage of USB over ADB, SCSI, and serial ports is that USB is a cross-platform standard. This makes devices for the iMac, and other modern Macs, just as cheap as devices for Windows-based PCs and creates a greater number and variety of choices.

At 1.5 Mbps (Megabits per second) in low speed and 12.0 Mbps (Megabits per second) in high speed, USB is also faster than ADB and serial ports, running at 10k per second and 320k per second, respectively. USB is much slower than SCSI, which hits a top speed of 320 MB (Megabytes) per second, but USB does have the advantages of being "hot-swappable", meaning you can hook up and disconnect devices while the computer is running. USB also supports up to 127 devices, and you don't have to worry about ID conflicts that could sometimes trouble SCSI-based systems.

Essentially, USB is designed to simplify the low-end Macintosh peripherals market consolidating ADB and serial devices into a single easy-to-use format.

Will all USB peripherals work with the iMac G3?

All USB peripherals will work with the iMac G3 provided that the MacOS already supports the device type or a driver has been written to be compatible with the MacOS. To be sure, if you purchase a USB peripheral make sure that it explicitly states compatibility with the MacOS version that you are running on the iMac G3.

When will I need a USB hub? Why?

For convenience, you will need an external USB hub when you intend to connect more external devices than you have ports. The iMac G3 has four USB ports, two on the computer and two on the keyboard. One port on the computer will be occupied by the keyboard, and one port on the keyboard will be occupied by the mouse, so you can still hook up two other USB devices simultaneously. If you need to connect more USB devices, for example, an external hard drive, a printer, and a scanner and you don't want to have to unplug one device to use another one, then you will need an external USB hub. USB devices cannot be "daisy-chained" like SCSI devices.

The iMac keyboard is a "bus-powered USB hub with two USB Type A ports." According to Apple's iMac DevNote, "A bus-powered hub as defined in the USB specification does not provide enough power to support a second bus-powered hub. A second bus-powered hub must be connected to a USB port on the computer, not to a port on the Apple USB keyboard."

So, essentially, if you get an external USB hub, plug it into the computer and not the keyboard.