This tutorial from the Hover Craftie Blog is an extract from their book on making an HTML5 game in Phaser.
"Tilemaps are layered maps created for games using images split up into tiles, generally 16 x 16 pixels or 32 x 32 pixels in size. These images are known as Tilesets. These are special types of images that are built to be split up into these tiled segments.
You can then select these tile segments individually to build your unique tilemap. As mentioned, these tilemaps can be built with several layers. For our game we will build a tilemap with two layers, one for the background and one for the platforms. By having this second platform layer we can isolate collisions between the platform tiles and any sprites we use."