All gutters are either sectional or seamless. Sectional gutters are sold in pieces and installed as component systems. All do-it-yourself gutter systems that you find at the traditional home improvement stores are sectional gutters. While it is not common some professionals install sectional gutters as well.
Sectional gutters have many connection pieces - snap in place midlines, end caps, corner pieces and drop outlets. The major drawback to a do-it-yourself gutter system is with the many seams as they eventually invite leaks.
Seamless gutters won't leak at seams because there are none. The different sections of a seamless system join only at inside and outside corners and at downspout outlets. That's why they're the most popular configuration. Seamless gutters are custom made on site to fit your installation to perfection.