Steadily increasing in popularity, mirrorless cameras have similar functionality to Digital SLRs but they come in a smaller and lighter package due to them not including a mirror in their build.
This portability makes them perfect for carrying around for longer periods, such as when you are travelling. Like Digital SLR cameras they have interchangeable lenses, although many brands do not have the same variety that a developed DSLR brand does.
Purists may be wary of their lack of optical viewfinder, but with electronic viewfinders improving all the time, this is not much a drawback. In terms of quality, Mirrorless cameras are up there with DSLRs of the same price point, with the DSLR still having a lead if you compare the higher-end models.