The brilliant Colleen Lea demonstrates how to sew raglan sleeves into a shirt. Collee is also the master seamstress over at Fashion Sewing Blog TV!
A raglan sleeve is a type of sleeve with distinguishing seams extending the sleeve as one piece fully to the collar, leaving a diagonal seam from underarm to collarbone. It is popular in sports, casual, and exercise wear.
Here is a pattern guide showing the detailed construction of a raglan sleeve. (link opens in a new window)