Rueda contratiempo is not really a separate structure, but it may be used in several ways and combined with other rueda structures (like dancing rueda combo with two circles and only one dancing contratiempo).
Rueda contratiempo is a mix between Son rueda and rueda de casino. Basically you dance regular rueda de casino with the Son timing (contratiempo timing).
‘contratiempo’ means ‘counter timing’, and refers to the fact that the emphasis is on beat 2-4-6-8 in stead of 1-3-5-7 (also referred to as atiempo).
The main difference between Son rueda and Rueda contratiempo is the figures used. But there are not necessarily clear boundaries, the same way that you may also use several Son figures in Salsa.
In practice when dancing Rueda contratiempo you would commonly use the Rueda figures that are closest to the Son traditon, like plancha/borracho, tornillo, roedo, etc.