Basically, WGS 84 is a single map projection represented in degrees, while UTM is like a local system, that divides the earth into 60 zones, 6 degrees of longitude each.
QGIS Docs has some great reference material on the subject you can read here: https://docs.qgis.org/testing/en/docs/gentle_gis_introduction/coord...
To quickly answer your question generally:
WGS 84 is a Geographic Coordinate System using angular units, a prime meridian, and spherical (earth like) datum. Geographic Coordinate Systems are excellent at representing point locations. As accurate as your measuring system is. However calculating distance and areas are complicated, and when viewed on a flat map like surface can result in greatly distorted figures and graphical representation.
UTM is a Projected Coordinate System with a defined 2D flat surface datum divided into a grid using linear measurements feet or meters for instance. Calculating distance is easy, and areas are quickly calculated and graphically represented realistically. The weakness of a projected coordinate system is that no point is truly located accurately, even if the error introduced by projection is extremely low and below allowable thresholds.