Flannel is a soft, woven fabric made from wool, cotton, or synthetic fiber. Flannel also often has a nap on either or both sides, which gives it a soft finish, and is typically decorated with a Plaid pattern.
The term “flannel” is of Welsh origin, but the fabric has grown in popularity all over the world. Due to its warmth and comfortableness, flannel is often worn during the winter months or in colder climates, and is a popular material for pajamas and other sleeping garments.
In terms of style and fashion, I would avoid wearing flannel. Flannel has the reputation of being something that's worn by lumberjacks, so unless you're going for the Davy Crockett look, stay away from flannel.
If you insist on wearing flannel outside of your bedroom, make sure that the fit is good. Loose or baggy flannel shirts just look sloppy, whereas a tightly fitted flannel jacket or shirt might be passable. Avoid flannel pants at all costs.
Another tip is to mix it up with the colors. Red flannel has been way too played out, but if you try a bold yellow or neon color, there will be less of the stigma attached to flannel (although making those colors work is a challenge).