Snapshots are automatically built every 1-2 days from the SVN trunk (development) repository sources by the buildbot. They are untested and might not work properly.
If long term stability and package availability is important, use the release builds from http://downloads.openwrt.org
Chaos Calmer 15.05 is currently the most recent release.
The snapshots can be downloaded from http://downloads.openwrt.org/snapshots/trunk. At certain times all platforms might not be available or up to date, due to builds failing.
Since the snapshots are updated frequently and development rapidly at times, packages from the currently available build might not be compatible with previous builds. This is especially true for kernel modules, which require the exact same build to install. If incompatibilities occur, the only option is to flash the current firmware and try again. In general, aim to install all needed "kmod" kernel modules immediately after the firmware flash, so that the modules are from the same snapshot build as the firmware itself.
The snapshots are built with a fairly minimal configuration, and by default does not contain a web interface (LuCI), which has to be installed separately (luci.essentials).