short runs - 1 - 10 t shirts of the same image
(as opposed to 1000 t shirts with the same image on)
Screen printing - a printing process using screens
DTG - Direct To Garment printing is when they print directly onto the fabric with what is kind of like a massive printer !
(If you want to quickly know their differences just youtube them)
The digital file just needs to be ready for print as it is,
If you give them a photocopy of your artwork they have to scan it in, possibly clean up the image, resize, edit it maybe etc etc, it all takes time and time is money.
screen printers usually have bigger minimums than DTG printing because their process is much longer (you will understand if you youtube it)
I've never used any of those companies before but I know how to screen print so I know a bit.
Well did you look at the prices on the sites?
Lots of variables - what method of printing, what type ink, how many prints, how many colours, what your artwork is like, whether you supply the t shirts, if they supply the t shirts then what type of t shirt is it etc etc etc
Also some places do heat transfers too which should be cheaper but the quality isn't as good. They look cheaper.
some people also buy inkjet iron on paper and do it themselves for fun so it just depends !
I will add that if you decide to go with screen printing and you have a choice of ink, choose water based it is MUCH nicer than Plastisol (plastic ink, cheaper but nasty)