In simple gathering, parallel rows of running stitches are sewn along one edge of the fabric to be gathered. The stitching threads are then pulled or "drawn up" so that the fabric forms small folds along the threads.
Gathering seams once involved tedious hand sewing of basting, which was time consuming and inefficient, especially with heavy fabric. Now, a quick and easy way to make a gather is to use a wide zigzag stitch with a sewing machine. Both the upper and lower thread are pulled long and placed in front of the sewing machine. Then zigzagging is carefully sewed over top of the two threads without catching the threads as it is sewn. At the end the thread is pulled and is then gathered. 
This photo shows a quick and easy method of machine gathering. This zigzag gathering technique is the strongest and most sturdy method of gathering.
Pleating or plaiting is a type of gathering in which the folds are usually larger, made by hand and pinned in place, rather than drawn up on threads, but very small pleats are often identical to evenly spaced gathers. Pleating is mainly used to make skirts, but can have other uses.(See main article Pleat.)
Shirring or gauging is a decorative technique in which a panel of fabric is gathered with many rows of stitching across its entire length and then attached to a foundation or lining to hold the gathers in place. It is very commonly used to make larger pieces of clothing with some shape to them.