H.O.R.S.E is a mixed game where multiple pokerr variants are played at set intervals.
The letters for these mixed games are made up from the following :

H: Texas Hold 'emm
O: Omahaa Eights or better
A: Omaha Hii
R: Razzz
S: Seven card Studd
E: Seven card studd Eight or better

There are several common variations of H.O.R.S.E mixed games such as H.O.S.E and S.H.O.E, but we try to offer some of the less played mixed games such as R.A.S.H., H.A.R, H.A. and H.O.
Games progress in order of letters shown starting with Hold'em then Omaha ect., at set timed intervals. For tournaments game type changes every time the blinds go up and for ring games every 8 hands. The exception to this is in H.A. and H.O ring games where the interval is set to every 10 hands.
For more information on each game type you can visit our pages on how to play each of these games