In SuperTuxKart, all karts fall under one of three categories/classes which produce different strengths and weaknesses for each kart, depending on its class.
There are three classes of karts, depending on their weight: light, medium and heavy.
This is especially useful at the start, after an accident or in tracks with a lot of sharp turns.
How fast the kart can go. High value here is especially useful in tracks with straight lines and gentle curves.
How much speed you get from a nitro can.
Lightweights / Lights
Middleweights / Mediums
Heavyweights / Heavies
The kart choice should always depend on the gamemode and track/arena being played. In Soccer, mostly lightweights are used and in Time Trial (Speedruns) heavyweights are favoured. Anyway, choice depends on the personal taste and the track. However, most used karts are middleweights that give a good balance between all factors.