Look at vs Watch

In this context look is usually followed by the preposition at.

When you look at someone or something you are interested in the appearance.

Generally we look at things that are static.

For example:

Look at these photos, they're really good.
I went to the art gallery to look at the exhibition of paintings.

Watch is a verb.

When you watch someone or something you are interested in what happens.

Generally we watch things that move or change state.

For example:

I watch TV every night.
The security guard watched the shoplifter steal the clock.

Note:

If I say to you "Look at him!" I mean for you to check out his appearance. But, if I say to you "Watch him!" I mean it as a warning.