Crawling (also known as "crouchwalking") is a technique introduced in Super Smash Bros. Brawl. While crouching, it allows certain characters to move while staying low to the ground, and can be used to avoid certain projectiles.
The usefulness of crawling primarily depends on how low the character's crouch is. For example, Snake has a very low crouch that significantly reduces his height, making his crawl highly effective for dodging projectiles and attacks with high hitboxes, while Bowser's crouch is much higher and barely reduces his height, leaving it near useless to dodge such attacks. While most characters can change the direction they face while crawling, all crawling characters can crawl backwards.
Characters can use their down tilt attack while crawling. Some characters have down tilts that move them forward, causing them to pseudo-crawl, but the usefulness of this is questionable.
Crawling is not used often in competitive matches as it is usually not that helpful nor practical for using over other methods of approach. The exception to this are characters with very low crawls (such as the aforementioned Snake) in certain matchups, as their low crawl allows them to approach while being unable to be hit by opposing projectiles, particularly against Falco.
Characters who crawl also have the advantage of crawl-sliding.
 List of characters who can crawl in Brawl
 List of characters who can pseudo-crawl in Brawl
The following characters can approach while using their down-tilts, which move them forward at a slow rate when used rapidly.
 List of known characters who can crawl in Smash 4