Most USB plugs do have a "seam" on the bottom of the connector. I've seen only one that has the seam on the top, on a "special purpose portable device."

Standard practice is to have the "USB Symbol" - that looks a bit like Poseiden's Pitchfork after a shipwreck - on the top of the connector, but on quite a few of my cables the symbol is almost impossible to see, even with a bright light and magnifying optical aids. (I buy some really cheap stuff, sometimes.)

The connector also, if inspected very carefully, will be seen to be very slightly wider on the top edge than on the bottom. This is sometimes with "slanted sides" and sometimes just with a very slightly projecting "ridge" on the top edge. Sometimes it's easier to "feel" the taper than to see it. The "ridge" style is especially common in alternate "mini" and "micro" connectors as used for cameras and other "intemittently connected" devices.

It may be necessary to distinguish between USB PORTS and USB SOCKETS. Although many computers have numerous sockets (holes to plug into), its common for all of them to be connected (inside the computer) to a single Port. The "port" is the "logical identity" that the computer uses to know which device to exchange data with.

Some computers may have USB connectors front and back. It's common for all of the front connectors to be on one port, and all of the rear ones to be on a separate one. But sometimes half of the front and back ones are on one with the other half of both on the other. (Your computer maker probably won't tell you which is which.)

The USB port is intended to be able to supply power to the connected device(s) but is limited to 500 mA per device. The "card" or "chips on motherboard" that manage the built in ports may, however, not be able to supply the full 500 mA to all of the connectors on the computer. Most USB devices don't need the maximum (500 mA) power from the USB port, but some do. External hard drives are among the ones most likely to need full power.

An external hub lets you plug the hub into one USB socket on the computer, and plug several USB devices into the hub. Hubs may be "passive" or "powered." For a passive hub, the computer connection can (should) supply only the 500 mA provided by the USB connector on the computer to run all of the devices plugged into the hub. Passive hubs usually are fine to take along with a laptop, just in case you need to read/write a flash drive or download from a camera; but for desktop or other heavily used application a powered hub is recommended.

With a powered hub, the computer supplies up to the 500 mA to the hub, for getting data in and out of it; but a separate "wall wart" transformer supplies the 500 mA (where needed) to the devices plugged into the hub. If you're connecting external hard drives through a hub, or just connecting lots of "stuff" there, a powered hub is recommended.

In principle, a single USB port can manage 8 attached devices logically - subject to the power capacity limits mentioned. A hub appears to the port it plugs into as a single device, and can itself manage (logically) 8 attached devices. Most computers, and most hubs, don't "push the envelope" and offer at most 7 connections to a port for devices. Four and five port passive hubs are probably most common.

Theoretically if you attached a 7-port hub to each of the 7 device connections on the computer's port, you could connect 49 USB devices. WinXP and later will handle the "logical management" pretty much automatically. Win2K should be okay, but is more likely to need some setup help.

Each hub that you connect requires the computer (look in network setup) to creat a "bridge" to block "circulating error loops,"¹ and connecting many hubs (too many bridges) is a possible way of bogging down the whole computer, so it has been advised that one should not exceed 15 total USB devices on a single computer. That advice came when USB-1 was still common, and you might be able to push it a bit more with USB-2; but it probably is still a good rule to follow.

¹ Each hub is, for logical/data-exchange purposes, a separate "network."

I've had up to 4 USB hubs connected temporarily in WinXP and in Vista, with no problems; but the total simultaneous devices I've tried hasn't gone past 11, I believe, at one time.

There is sometimes a slight transmission² delay when communicating with a device through a hub, and Plug-N-Play may occasionally have difficulty identifying a device that's on the "other end" of a hub; so certain kinds of devices should still be plugged directly into the built in computer port(s) where possible. USB keyboards and mouses, and bidirectional printers that send data both ways to/from the computer are some that usually will recommend (in setup instructions) that the USB connection should be directly to the computer.

² The delay may be because more than one device is "talking" at the same time, and the hub input/output for one may, intermittently, have to wait until the other device shuts up.

When USB-2 began to be popular, it was warned that connecting one USB-1 device to a USB-2 port (with USB-2 devices on other connectors for the port) required that all external USB devices communicate at USB-1 speed. Current USB-2 devices mostly can select the drop-down speed for the USB-1 device(s) without slowing down the USB-2 ones; but you may still find some accessories, especially passive hubs, with the one-speed-for-all limitation. If you have USB-1 (originally just labelled USB) devices, you probably should check to see if a powered hub you're considering allows simultaneous separate speeds. (But if doesn't matter if the computer doesn't provide USB-2 ports.)

John