There are many different types of layers that can be added to a QGIS project.
The following list is not exhaustive but should give an overview.
More descriptions will be added as time permits
- Simple raster. There are many different raster formats supported, but possibly the two most common are likely to be TIFF (used by Ordnance Survey) and JPG (typically for photographs). without an associated world file (a TFW for Ordnance Survey), these images will neither be correctly positioned nor transformed to match the project’s CRS. It is possible to load several raster tiles as individual layers, but for both convenience and render speed, a virtual raster is a better solution.
- Virtual raster. This uses a VRT file to define a group of simple rasters and how they fit together to appear as a single layer. If you wish to use several tiles to create a single larger image, you will generally be better doing so with a virtual raster.
- Delimited text.