UDL in beams

Hi all,
I'm a first year Mechanical engineering student.The question is regarding strength of materials.I'm having problem with understanding the concept of Uniformly Distributed load.It was said that a beam's self weight is UDL for the beam.What is the difference between Pressure acting on beam and UDL on beam.In ANSYS we have both pressure and UDL on beams.So are they same or different?.

Sorry if I have asked something very elementary.But I need to understand this.Also pls give some references where i can get a clear idea.

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