Shiplap joints are formed by cutting identical rabbets into opposite faces of adjoining boards. This produces a joint where the rabbets overlap, preventing gaps between the boards from being visible. Shiplap joints are often referred to as a poor-man's tongue-and-groove since the visual effect is very similar to tongue-and-groove, however less work is involved.
The area where shiplap joints are inferior to tongue-and-groove, though, is that shiplap joints do not keep the boards flush with one another. However if the boards are to be fastened at regular intervals along their length and the wood is relatively stable, shiplap joints can save you much time.
To hide any uneven boards, you can add an edge detail like a chamfer of bead to the end of each board, or you can cover the exposed edges with a decorative molding.