The Ulna, in my opinion, is one of the most interesting looking bones (besides things like the ethmoid and sphenoid bone). It is exceptionally top heavy for being so thin, and the various landmarks of the proximal end looks a little weird. Anyway, it is one of two bones that make up the osseous structure of your forearm, the ulna is medial to the radius (in correct anatomical position), and it is the radius that crosses over the ulna when you rotate your wrist or turn your arm around. I won't attempt to draw an ascii ulna (I know, you must be sad...), but the bone does deserve a little more attention than the dictionary definition listed in this node.
To begin with, at the top we have a large violin scroll-like process called the olecranon process. This is where the ulna articulates with the elbow joint. On the medial edge of this is a concave groove called the semilunar notch (it has a crescent shape) and the bottom culminates in a protrusion called the coronoid process (not to be mistaken with the corocoid process - which is on the innominate bone).
Medial to the coronoid and right below the olecranon is another little groove, called the radial notch. This is where the radius articulates with the ulna. On the shaft of the ulna there is an interosseous crest, which is the attachment site of the connective tissue between the radius and ulna which allows one to rotate their forearm. Interestingly enough, the distal end of the ulna is called the head, which sits next to a groove in the radius called, amazingly enough, the ulnar notch. A posterior and medial styloid process which connects only indirectly with the wrist.
This styloid process is the most common feature used when "siding" the bone - ie, figuring out which side of the skeleton it belongs on if it is disarticulated. This is because this process is marked by a groove where the extensor carpi ulnaris muscle passes through. When you hold the bone upside down and look at the styloid, this groove will be on the side that the bone belongs to. This is similar to the method of siding the fibula.
, "Human Osteology, 5th ed."