They include jersey or single knit, interlock, double knit, rib, waffle, pointelle, and pique. The most common knits in shirts are interlock and jersey. Both are usually smooth with interlock knits having a bit more body. The weight can be varied by using heavier yarns. Sometimes surface yarns are brushed for a softer finish and additional warmth to trap body heat and act as an insulator.
Double knits look very similar to interlock knits but are heavier and more stable.
Rib knits are very stretchy and form fitting. They have lengthwise ridges throughout the fabric. Popular for cold weather knit shirts is a waffle stitch. These shirts can be very dense which traps body heat and makes a warmer garment. Thermal shirts will often use this knit structure especially combined with a brushed surface next to the skin for additional warmth.