Please note, however, that cropping only trims your video sides within the video frame, as ATR's illustration shows. It does not change the size and shape of your actualy video frame, the way cropping a photo in Photoshop does.
Just an add on....
Depending how far you get involved with cropping in Premiere Elements, you might be interested in a blog post that I wrote on Effects Mask in which a cropping application is discussed.
Also, there is a zoom option in the Premiere Elements Crop dialog to consider. The Adobe Effects References documents includes
The Crop effect trims rows of pixels from the edges of a clip and, if you select the Zoom option, automatically resizes the trimmed clip to its original dimensions. Use the slider controls to crop each edge of the clip separately. You can crop by pixels or clip percentage.