You decided to download a pre-built binary firmware image. Good. You can choose between a firmware based on stable sources or one based on bleeding edge sources? We recommend stable. You can also choose between JFFS2 and SquashFS firmware images. We recommend SquashFS.
Although the trunk images offered for download are usually pretty stable, OpenWrt is not a rolling release. There is always a stable version. It is of course possible that your device or the hardware version of your device, is not supported in the latest stable. There will be no backports, you have to use bleeding edge in that case.
Latest OpenWrt release
Outdated OpenWrt releases
If there are different image files for different hardware revisions, you have to use the one for your specific hardware. Different revisions can mean completely different hardware! It is also possible that newer hardware revisions are only supported in the trunk or not yet supported at all. If your version is not explicitly supported you should ask in the forum. Then come back and update the wiki!
Sometimes there is a image file with the word factory in the name. If there is one, you have to flash this one over an OEM firmware (aka factory firmware).
Sometimes (depends on the maintainer) two different versions are offered for download: JFFS2 and SquashFS. We recommend SquashFS
Usually an imagefile with the word "sysupgrade" is offered as well. This is for sysupgrading