Some people do knit tube socks, but for the best fit socks have to have a heel. If you think about it, the leg and the foot of a sock are both tubes, but they run in different directions, so the heel has a lot of work to do: it turns the knitting and makes the finished sock snug on your foot.
There are many different ways to navigate the turn that your knitting must take when joining the leg to the foot on a knit sock, but one of the most popular -- at least when it comes to knitting toe-up socks -- is the short row heel.
Short-row heels can also be worked on cuff-down socks, and either way they provide a nice solution to the problem of having a complex stitch or color pattern that runs down the cuff and the foot that you don't want to have interrupted by a gusset that requires increasing or decreasing stitches.
Whichever direction you're going on your sock, the short row heel begins with working a portion of the heel stitches (which are typically half the number of stitches used in the whole sock) and performing a wrap and turn to work over some of those stitches again without working the entire row.
Short rows are a wonderful technique because they help provide more space in part of the project -- in this case, forming a heel cup -- without making the whole project longer. The wrapping of the stitch is included so there are not holes on the sides of the heel that show where you turned the work within the row.
Once you've worked several rows with progressively fewer stitches, you will begin working rows that are progressively longer and will work the wraps together with their stitches to hide the extra bumps they make.
You can see the short row heel in action on my Rib Fantastic Socks, which are worked from the cuff down but use this style of heel so that the stitch pattern is not interrupted and you don't have to work out the math of a gusset around the stitch pattern.