Screen printing is a printing process usually used for larger orders of printed t-shirts and larger designs. Screen-printed t-shirts tend to keep a good level of quality and maintain their colour and feel for a very long period of time. Most printing you see in the High Street shops will have been printed this way. Although other methods are becoming increasingly more popular, this is largely due to the amount of customers looking for photographs on shirts.
This process should not be confused with heat transfer printing or vinyl printing. These methods can capture much higher detailed designs, like photographs. But will simply not last anywhere near as long. They have a tendency to crack too.
When screen printing, only one colour at a time is printed, meaning the cost will increase with each additional colour applied. For each separate colour an additional screen is required, so a design with 4 colours in it will need 4 screens, often this can mean other printing methods are considered on lower quantities.
If you require a job with a quantity above 25, with minimal amounts of colours, screen printing is highly advised.