Direct To Garment is a process that uses a special printer, generally a variation on an Ink Jet printer, along with special ink, to print directly from a computer to a t-shirt. This special DTG printer is customized so that you can feed one t-shirt at a time into the printer and it will spray multiple ink colors on the shirt similar to printing ink onto a piece of paper. This has certain advantages, but is only viable under certain circumstances.
One of these is that DTG is generally used primarily for small quantity runs when you have a lot of colors to print. This is because it takes a long time to print each shirt, as compared to traditional screen printing (an automatic screen printing press can print 10 to 20 times as many shirts in the same time it takes to print one shirt with a DTG printer).
Another issue is that DTG can be expensive. One DTG printer can cost anywhere from $11,000 to over $200,000. And a major drawback is that most DTG printers only print on white or very light colored t-shirts. There are some DTG printers that can print on dark shirts, but they naturally cost more and take longer to print a single shirt.
Probably a major consideration, besides the cost, is that unlike traditional screen printing which prints on to the shirt (and bleeds into it), DTG prints directly into the shirt fabric (which partly gives DTG its name). You might think this would be a good thing, however, the problem is that as the shirt fades so does the DTG design (because it is part of the fabric). Also because of the nature of DTG printing it tends to lack opacity and the same brightness that you would see in a traditionally screen printed design.
DTG has its purpose in the screen printing industry, but under most circumstances traditional screen printing techniques will provide a brighter and lasting print. For more detail on how traditional screen printing works check out What is Screen Printing & How Does it Work?