From Simple Modern Sewing:
Raglan-style sleeves are easy to sew and very comfortable to wear. The sleeve's diagonal seam runs from the garment's neckline to the side seam to create a large and loose fitting armhole. This sleeve style is perfect for casual clothes and for tops you want to wear under layers. Experiment with different fabrics to create different looks. If you're just learning to sew, start with Pattern 2a. The frilly neckline is finished simply with a strip of elastic tape within a fabric casing.