Many people confuse the words "gallery" and “museum,” or use them to mean the same thing. And no wonder the meanings can be confusing!
Here's one big difference: Although both galleries and museums hold works of art, a gallery usually sells the art it displays. Galleries are often small spaces, and they're generally private meaning that they’re owned by one person or a small group of people. They often show the work of one artist, or a small group of artists on a particular theme.
That having been said, some museums are actually called galleries! The National Gallery of Art in Washington DC and the National Gallery in London are actually museums--you can’t buy the paintings on view (although you could probably pick up a souvenir in the gift shop!). The word “gallery” here often means a museum that is publicly owned.
To make it even more confusing, the rooms in a museum where art is displayed are often called…you guessed it, galleries.
So if you want to go somewhere just to look at works of art, go to a museum; if you’re thinking of buying something, you would go to a gallery!