There are 10 watch faces to choose from on the Apple Watch, and many are customizable. My favorite might be Astronomy, which shows your actual location on a spinning Earth.
Astronomy also shows you the moon in its proper lunar phase: you can spin the crown back and forth to look at its phase by date.
Tap another corner, and the alignment of all the planets in the solar system becomes your watch face: spin the crown and find out when Mars will be back near Earth again.
One of the few truly digital-style faces on the Apple Watch, Modular shows many bits of information in an easily-readable format, like a dashboard.
Simple, and meditative: various flowers spring open, and you can read the time and date.
Another Motion customized face features hypnotizing swimming jellyfish: one of my personal favorites. You can also choose a butterfly-only watch face.
Solar shows the sun's actual position, and an arc of the day's length based on where you are. Find out when sunrise or sunset are, and spin the crown to see the pretty animations.
A basic round watch face that can be customized with different dials, and information in the corners to show battery life, fitness progress, alternate time zones, and more.
Just the time: you can pick the color you want those giant numbers in, but you sadly can't change the layout.
It's Mickey Mouse! It animates just like a classic cartoon, tells the time, and hints at possible Disney faces to come.
Another customizable face that has bright, poppy neon colors.
Clean and basic...and, another round watch face.
A stopwatch that can be customized to show different spans of time around the dial.
Most watch faces allow some color, layout and feature-based customizing, within limits. Here, I'm picking the color highlights on one face.
"Complications" are the watch term for those bits of extra information around the watch faces. Apple has its own collection of data you can pick from (battery life, fitness, calendar, date, time zones, lunar phase), but third-party extensions aren't currently allowed.