Grow: Grow works by selecting more pixels that match the color of your selection. For example, you’ve selected a portion of a tree. Grow will choose all of the pixels that match the color of the tree portion you’ve selected. This works best if you have an image that has a high contrast, otherwise it may select more pixels than you need.
Similar: Similar will select all of the pixels in an image that match the color of your selected pixels. Again, this works best with high contrast images so that you don’t end up selecting pixels you didn’t need.