A udeb (or micro-deb) is a special kind of Debian package.

Technically udebs are very similar to regular packages: you can look at their contents using dpkg -c, and extract them using dpkg -x and dpkg -e.

The main difference is that a lot of policy requirements are waived. For example, a udeb does not contain a changelog, licence, manpages or md5sums[5]. The reason is to minimize size which is important as the installation takes place completely in RAM, with swap only becoming available after stage 4 of the installation (partitioning).

Another important difference is that udebs are not really meant to be uninstalled or upgraded.