Knit shirts or sweaters are made out of thread or yarn, just like woven things, but instead of being made on a loom they are made on the pragmatically named "knitting machine." Actually, there may be an industry term for the very large knitting machines they use, but I haven't heard of one.
Instead of threads running at 90' to each other like woven fabrics, knit fabrics are made of looped threads; each row of loops interlocking with the preceeding and subsequent loops of thread. Because the thread is in loops instead of pulled straight, knit fabrics have more ability to stretch than most woven fabrics. (Technical note: ability to stretch is called "ease" or "give".) If you are unfamiliar with the concept of knitting, take a look at your socks; they are knitted. If this doesn't help, find a friend who does crafts and ask if they can give you a demonstration of knitting.
Those of you who are familiar with the ancient knitting method involving knitting needles (sticks) done by a person using two hands would be quite mystified by the modern knitting machine, which uses multiple little hooked needles to keep many rows of loops in play at one time. I don't know a lot about it; I just know that I'm mystified, too. There are tabletop knitting machines which some people buy as either a hobby or as a way of earning extra money; these days if you see a "handmade" sweater in a store, there's a good chance it was made on one of these home machines rather than by knitting needles.
At any rate, the knit fabrics used in work shirts are widely varied these days. Fortunately, the days of polyester knit plaid slacks with sewn-in front creases are mostly a fading memory. Good quality clothing in cotton and wool, with a wide color selection, are available from many sources.
When you are shopping for knit shirts, you will have a choice of several major varieties of shirt: Turtlenecks, Mock Turtlenecks (the same as a turtleneck, except the collar is lower and looser) shirts with buttonable collars, such as Polo or Rugby shirts, T-shirts, sweaters and sweatshirts. For those of you that aren't techie-types, the Turtlenecks and collared shirts are the ones most appropriate for work, but some of the fabric types migrate across the styles. I'll list them beginning with the most likely to be used at work.
Interlock: Most shirts that are cotton interlock (or cotton varieties, like Pima) are a lightweight fabric with a smooth finish. There are some other terms that also apply, but they are all fairly similar. These fabrics look the most un-knitlike of the group. T-shirts (so named because they are T-shaped when laid flat) are also made using a relatively flat smooth finish.
Pique [that's french; pronounce it pe-kay] or Mesh: This fabric seems thicker than the interlocks, but it is often about the same weight. Instead of being knitted flat, the stitches form a sort of waffleish weave--more room for air to move around, while still being opaque. Polo shirts are often made of this fabric.
Doubled knits: As in sweatshirts, that have a smooth exterior and fluff inside, the fabric is one unit, but has two sides to it.
Special knits: manufacturers will experiment with different patterns in the knits, producing classics like cables, herringbone, ribbed effects, or stripes. Others use colored thread to make plaid knits (different than knit fabric that has been *printed* with a plaid) or other visual effects.
Specific things to pay attention to in sweaters:
Whatever special effect is done on the front of a shirt or sweater, you will look much better if it is also carried around to the back of the shirt. I have seen a lot of "cable" sweaters that are nicely cabled in the front (that's a rope-twist pattern, btw) and have a plain knit on the back. It makes you look half-finished.
One of the hazards of poor sweater construction is "floaters." If you have a sweater with multiple colors in the design, there are areas in the fabric where one or more color won't be needed to enhance the design. In these cases, that thread is carried along on the wrong side (inside) of t