What is MWD & LWD ?

MWD stands for Measurements While Drilling.
LWD stands for Logging While Drilling.

As a well is drilled to a subsurface target, some instruments are attached to drilling string just above the drilling bit. The MWD tools measure the well trajectory parameters like inclination, azimuth etc, while LWD tools measure rock properties like Gamma Ray, Neutron & Sonic Density etc.

In modern highly complicated reservoirs, these tools play vital roles in steering the well along the most favorable reservoir body. It also saves time and money eliminating need for wireline logs and round trips of the drilling bit. 

Another very interesting aspects of these tools is that they communicate with the engineer sitting on the surface via mud-telemetry, that is, they uses pulses in the mud or the fluid system present inside the drill string to send data to the surface and also to receive instructions from the surface.