Neither of us knew the specific answer. Yes, flannel is softer and warmer (think flannel sheets vs cotton sheets or a flannel shirt vs cotton shirt). I thought that flannel was a type of cotton but, what else?
Flannel was based on the Welsh term "gwlanan" which means wool... Today, it means any brushed fabric. Cotton is a fiber used for many fabrics and some cotton fabrics have a brushed finish and are called flannel. The brushed finished is also called a "nap". (Other fabrics with a nap are velvet, think of how the texture as opposed to cloth).
Some flannel fabrics are made from things other than cotton - so I was wrong-- flannel is not just a type of cotton, although it can be a type of brushed cotton.
The brushed finish on the flannel is made by scratching and raising the nap with a wire brush like thing. Flannel can have a nap on one or both sides.
Flannel can be 100% cotton, wool or a cotton and synthetic blend. When looking for flannel pajamas or sheets for warmth, be sure to look for 100% cotton on the label. The cheaper flannels have the synthetic material - rayon, nylon or polyester added.
Some people think that flannel refers to the plaid pattern. It does not, plaid is a pattern, flannel is a type of brushed fabric. Many flannel shirts are plaid, but not all plaid is flannel....
Flannel that is sold in the US and known as flannel to us is called flannelette in the UK by law. Flannelette was used mostly by poorer classes and used almost extensively to keep warm (as in undergarments and clothing for outdoor work, farming & lumberjacks). Today, many people in the same fields use flannel for the same reasons.
Flannel has become a fashion statement since the 1990's when "grunge rockers" in the Northwest incorporated plaid flannel shirts into their wardrobes. Many young people followed suit and flannel became used for more than just utilitarian uses.
Flannel comes in all colors and patterns.