There are various modes and the Alta HR can be set to record heart rate automatically or manually and to background sync etc. The different options will affect battery life, which is about 7 days on a full charge.
The Alta HR will track steps (and a run if you're inclined that way), distance travelled (but it's calculated as there's no GPS), calories and sleep (if you wear it to bed). If you do wear it to bed it will also calculate your resting heart rate as well as show your type of sleep (light, deep, REM and awake).
Though splash proof, it's not waterproof so no swimming or showering. When you remove the unit it should stop trying to read your heart rate, but sometimes it seems to continue to try to read it (the LEDs flicker) for quite some time.
The smarts is in the Fitbit companion app (Apple Health compatible on iOS) and that where most of the information is displayed.
There are a variety of standard bands that can be bought in different colours. They're made of a fairly chunky silicon that 'feels' pretty solid and use a standard pin/hole clasp to close so can easily be adjusted for size.
Fitbit are now selling premium leather bands and even metal bands which turn the Alta HR into bracelet, though they're pretty expensive and as they're solid, heart rate tracking may not work (as the unit may not be snug on your skin).
Fitbit make other trackers that are more functional, but the Alta HR is definitely the prettiest, but it's reasonably expensive at £129 or more for the premium versions. It's a lot compared to something like the Mi Band2 which can be had for about £16 (it's not as pretty) for almost identical functionality and the battery life on the Mi is over a month.