Purlins are beams that run horizontally and thus parallel to the gutter.
The purlins are supported by truss or walls. It is highly important that these purlins are tightly mounted to the wall. This is necessary to create a solid construction.
You have ordinary purlins, which are suited to use diagonally of the roof. Another time of purlin, is the ridgepurlin, which is used above the ridge.