Using masks to exclude part of a still image

Hi,

I have a still image the at brought into AE.  I would like to use only part of the image and exclude the rest.  In photoshop you can create a mask and place the object in the mask on a separate layer.  Is there an equivalent process in AE?  I know I can go back to PS and extract the part of the image I want, but I thought there might be a way in AE.  Thanks.

Cliff