Graphic artists typically associate cropping images with photo-editing software. Adobe Illustrator CC, as a vector graphics program, doesn't have a designated crop tool, but you can crop images and graphics in Illustrator using masks and artboards.
Cropping in Illustrator
Because Illustrator lacks a crop tool, the act of cropping is more about hiding what you don't want to see in Illustrator than selecting what you do want to see. Instead of dragging a cropping box to select the area you want to keep, like you do in most photo-editing programs, you focus on what you want to get rid of. You're not limited to a square or elliptical shape. You can use any shape -- one you draw with the pen tool or one you create with a shape tool -- to design a crop on a photo or other image.
Creating a clipping mask is quick, but it doesn't give you as many choices as using opacity masks. To crop with a clipping mask, choose "Find Edges" under the "View" menu to surround the image with blue lines. Click on the image with the selection tool to activate "Mask" as an option in the top control bar. Click on "Mask" to create the clipping mask. Select the "Edit Clipping Path" button in the control bar and drag the blue lines where you want them. To move one side at a time, use the direct selection tool to click on the open boxes on two adjacent corners. Use the arrow keys to move the selected line inward to your desired location. The result looks like part of the image is being cropped away, but it's just hidden behind a clipping mask. Choose the "Edit Contents" button to move or make changes to the image within the clipping mask. To remove the mask and see the entire image, choose "Object," "Clipping Mask" and "Release."
Opacity masks work in a similar way to clipping masks, but they offer more control. Start with a fill color of white and no stroke color. Draw the desired crop shape -- which could be a circle, rectangle or custom shape -- over the image. Select the shape and press "Shift" while clicking on the image to select both at the same time. Under the "Window" menu, choose "Transparency" to open the necessary panel. Click "Make Mask," which crops the image to the outer edges of the shape. Click the chain link button between the image and shape icons on the Transparency panel to manipulate the elements separately. You can change the opacity of the mask, the color or add color to the stroke to make a border around the image. Click "Release" on the Transparency panel to undo the mask.
Using the Artboard
An artboard is similar to a canvas in a photo-editing program. It shows you the printable area of the image. After clicking on the artboard tool, which looks like a square whose lines extend past the edges, click on the image and drag a box to select an area. Adjust the corners or move the box around until the artboard box is the right size for your needs. Press "Enter" to activate the artboard. To save the cropped image, go to the "File" menu and select either "Export" or "Save for Web," making sure the box beside "Use Artboards" is checked in the dialog box. The crop takes effect on the exported version; the entire image appears in the original version of the file.