It was basically an ankle-high boot with a wedge-block heel that zipped up the side, and usually fit tightly against the foot. Originally developed from a type of equestrian boot that was a part of traditional English riding outfits, during the mid-60's they started to creep into the increasingly more outlandish garments worn by fashionable young people in the cities.
The Beatles quickly popularized a type of Chelsea boot, known commonly as the 'Beatle boot,' which ended higher on the leg and sported a sharper toe. Chelsea boots of both types quickly became popular with mods because it fit in with their sleek, tailored look. Some varieties sported the so-called 'Cuban heel,' which was supposedly derived from flamenco footwear.
These days, one hardly sees this cool sort of footwear unless somebody's going to a costume party dressed as Austin Powers.