A combination boiler is so-called because it has two heat exchangers, one for space heating and one for water heating.
In these systems, hot water is generated as it is needed. There is no storage of hot water and hence no energy losses due to storage.
These boilers are most applicable where a small number of people are using the system, the house is small and multiple use is infrequent. The drawback of these systems is that only a limited amount of hot water can be drawn off at any time.