Underbalanced drilling is a drilling technique that hydrostatic pressure from drilling mud is less than reservoir pressure. The underbalanced drilling can be created by using low weight fluids as base oil, fresh water which has less hydrostatic pressure than the expected formation pressure. Additionally, this drilling method is achieved by using low density drilling fluids as gas, foam, combination between conversional drilling fluid and foam/air.
The main advantage of Underbalanced drilling is to minimized formation damage in reservoir. With underbalanced drilling, the reservoir fluid is allowed to flow out into the well therefore the chance of mud invasion into the reservoir is minimal. Hence, the production from the well from this technique is higher than normal wells. Conversely, drilling with conventional method (overbalanced drilling) creates near wellbore damages which affect hydrocarbon production.
The underbalanced drilling is often applied to horizontal drilling wells because long horizontal reservoir will not be damaged with drilling fluid. Only few inches of near wellbore damage in the horizontal section can drastically reduce the reservoir performance.
This video from Shell demonstrates you regarding underbalanced drilling and there is one section showing comparison between conventional and underbalanced drilling. This short VDO will definitely give you clear picture of this technique.