What is meant by 'Transactional Updates'

A "transactional update" is a kind of update that:

• is atomic -- while the update is running, your system is perfectly functioning;
• can be rolled back -- if the upgrade fails or if the newer software version is not compatible with your infrastructure, you can quickly restore the situation as it was before the upgrade.

With APT, generally only the latest version of a software is available (you can't roll back) and upgrading a package with all its dependencies may leave the system temporarily unusable during the process.

In addition, Snappy packages are "self-contained", that is: they have no dependencies. All the libraries and the components needed are bounded inside the same Snappy package.