A single bet is a bet on only 1 single odd and/or outcome.
A combination bet is one where several bets are combined on a bet slip; all selections must be correct before the bet pays out. Each odd you add to the bet slip multiplies the current odd. If you select two selections with an odd of 2.00, the total odd of the combination becomes 4.00. If you add another selection with an odd of 1.50, for example, the odd rises to 6.00 (2.00x2.00x1.50).
A system bet makes it possible to quickly create multiple combinations. For example, with 3 different selections you can very quickly bet on all possible doubles (a combination bet with 2 selections) within these 3 selections.