Vocal percussion is imitating drums and percussion with the voice. This technique was developed in the eighties in hiphop music. Actually, in this music style it is called beatboxing. In the a capella world however it is called vocal percussion.
With this concise history you get a good image of the period in which the groups were active, how long they played an important role and what influences they might have had. (The most fun is actually not looking at such an overview, but creating it. I love making lists.)
Whenever you are singing fast, energetic songs, the music will sound good if you make small accents on the notes. In this way you are emphasizing the rhythmical character of the music. We might call this ‘percussive’ singing. It is opposite to the classical way of legato singing.
The term ‘legato’ is common in classical music. It indicates that you should bind the notes, preventing gaps between them. But at the same time the term is used to indicate that the notes should keep their volume, creating a continuous sound. We might depict legato like this :