Saplings are a Decoration Block. In Creative mode, they can be placed freely. Saplings will grow into a full tree if the server has "grow-trees" set to "true" in the "server.properties" file. In Survival, Indev and Infdev, saplings appear randomly as tree leaves decay. In order to maximize your chance of finding a sapling, ensure that the leaves are not touching any wood or ground blocks. As the leaves decay, they will randomly drop a sapling, which you can come back and collect. The saplings can be replanted to make a new tree, but will only grow if they're on a lit dirt tile and have at least 4 blocks of "headroom" above them.