A marker is essentially a bigger, juicer pen. Most markers have a reservoir of soluble ink that is applied to a drawing through a felt or nylon tip. Markers come in several incarnations: art, studio or layout markers; marking pens; paint markers; calligraphy markers; etc. Most art markers consist of a broad nib on one end (excellent for filling in large areas) and a fine nib on the other (better for more precise work). Many also feature a third ultra-fine nib for tight detail work. Many times nibs are removable and replaceable in various shapes and sizes allowing the artist to create a range of marks.
Markers work a bit like watercolors. It's best to start light and build up to your colors since it can be tough to lighten a color once you've gone too dark. You can use a colorless blender to blend and soften the edges of your work. Depending on the surface, markers tend to dry quickly so don't plan on making color changes. Markers are appreciated for their immediate and colorful results, but the artwork is not intended to withstand the rigors of time.