Posted in other thread but in case you miss it there.

There is a button to tab target however it works slightly differently than other MMO games.

In ESO it will make the enemy have a white glow around them, you will be able to see and follow this glow even through walls and other terrain (very useful in PvP). This tab target doesn't lock all attacks onto a target however but does guide them, if you look away at another NPC clearly enough, or someone blocks your direct line of attack your attacks will hit that target not you tabbed one.

It is still very helpful for bosses etc where they have adds running around and you want to try and guide as many attacks as possible to a specific target.