Ashi Sabaki (Foot Movement)
In karate as other Japanese martial arts (Budo) we use different foot movement for different forms of attack and defence.
The 3 main types of footwork
If the back foot passes the front foot, it is called ayumi ashi. It is simple like walking forward or backward
Yori ashi (sliding feet)
The closest foot to the direction you wish to travel moves first then the following foot without crossing. Often used with front hand strike.
Yori Ashi Video Demo
Tsugi ashi (Replacing feet)
One foot replaces the other before that foot can move. So that the trailing foot moves to the front foot then the front foot advances or the front foot retreats to the trailing foot then the trailing foot retreats.