The Apple USB Mouse was Apple's first USB mouse, which made its debut with Apple's initial release of the iMac in 1998. While it was formally branded the "Apple USB Mouse", no one seems to call it that. Instead, most people choose to call it (most commonly) the "hockey puck mouse" or in some cases simply the "iMac mouse". It should be noted that the latter isn't a very accurate name since the Apple USB Mouse also saw action on Apple's USB-equipped Power Macs of the day, until Apple replaced the design with a larger optical USB mouse, the Apple Pro Mouse.
The Apple USB Mouse is approximately three inches in diameter, which is, coincidentally, just about the same width as a hockey puck. Although "hockey puck" probably isn't the best descriptor of the mouse's overall shape, since it isn't quite as thick as a hockey puck and (most importantly) the top isn't flat. Its shape is better described as being similar to the top of a mushroom or perhaps a muffin.
The mouse's casing is mostly translucent white, but the left and right sides of the mouse matched the color of the computer it accompanied (for example, with the first iMacs they were Bondi Blue). The cord is partially translucent and approximately two feet in length.
There is an Apple logo impressed dead center on top of the mouse, directly beneath this logo is the mouse ball (which you can see due to the translucency of the mouse). The mouse ball Apple shipped with the first iMacs was half blue and half white. After the introduction of "fruit flavored" iMacs (Revision C), Apple changed the mouse ball colors to dark grey and white. When the mouse is moved, if one's hand is not directly on top of it, one can see a delightful, hypnotic swirling through the mouse's casing as the ball spins.
One of the minor issues with the mouse's design was that because the button was so wide and smooth, and the mouse is round, the user's hand would slip off center and as a result the cord would twist or the mouse arrow would move in an unnatural way. Apple responded to this complaint with the final iteration of the Apple USB Mouse, which added a groove to the mouse button to allow the user to feel if the mouse was oriented correctly.
The biggest and most widespread complaint with the Apple USB Mouse was that it was simply too small for the hand of the average user. Some feel the small size was an intentional effort to make the machine more approachable for a target demographic of children and women that have smaller hands.
The Apple USB Mouse does require a different grip than most people are familiar with. Instead of cupping your entire hand around the mouse, you are supposed to lightly grip it with your fingertips. This grip tends to encourage you to move the mouse with your forearm instead of your wrist, thus reducing repetitive strain injuries to the wrist and hand. The mouse is much more ergonomical and feels much better to use when you hold it like this, however its size and shape still make it feel unique. Whether you like this feeling or not is a matter of preference.
The small size would have been less of an issue if more of the people who complained knew how to properly grip the mouse. The fact that the required grip isn't immediately obvious may be argued to be a design failure. Certainly the design didn't omit ergonomics when used properly.
Nevertheless, Apple (in true Apple fashion) did take these complaints to heart, but didn't respond to them until the component costs for optical tracking technology came down enough for them to respond with the radically new style enjoyed by the Apple Pro Mouse, which was a complete replacement for the Apple USB Mouse.
Note: ccunning contributed the part about how to properly grip the Apple USB Mouse.