Blanket insulation is one of the best kinds of insulation for do-it-yourself home projects, and comes in two forms: batt and roll.
The least expensive and most common insulation types, both are made from layers of fluffy fibers, usually fiberglass. Other common materials include rock and slag mineral wool, plastic, cotton, and even good old fashioned wool.
Both batt and roll insulation come in standardized widths to fit traditional stud, truss, joist, and rafter spacing, and both are available in faced and non-faced varieties (facing can make installation easier and sometimes adds extra flame resistance). Some people prefer rolls for seamless covering. Batts come in smaller chunks, which makes them better suited to awkward spaces that require cutting insulation to fit the surface.
Insulation effectiveness is measured by R-value, which correlates with the density and thickness of the product. In warmer climates, you can get away with a lower R-value, which will likely be cheaper, though a higher one will help retain the cool air coming out of your AC unit. In general, higher R-value insulation contributes to higher resale value.
For safety, always use mouth covers when installing blanket insulation. It's made of tiny particles that can be dangerous to breathe in. And remember to seal any exterior gaps or holes before applying insulation. As great as it is, it's not a replacement for a wall.