Adrenaline is an ePSP hack for the PSVita that was developed by TheFlow. It is also one of the most popular pieces of software for hacked Vita/PSTV consoles since it allows users to play PSP games and use homebrew while offering a whole slew of other features. Adrenaline has the following features:
Support for all PSP/PSX games, even compressed PSP ones.
Support for all types of homebrew including those that require 64MB of RAM
Save States

As you can see, Adrenaline gives you the full XMB experience which you can further customise to your liking!
Plugin support
Ability to use different partitions for PSP files (ux0,ur0,uma0,imc0)
Many other features including the ability to use the right analogue stick (with appropriate game specific plugins) and internet support
What got updated in Adrenaline-6.1?
Adrenaline-6.1 brings about the following:
Support for firmware 3.65 and 3.67.
This release is very important for those who have gone ahead and updated to firmware 3.65 with TheFlow’s recently released CFW updater. Adrenaline-6 will not work on firmware 3.65 or 3.67 due to the different offsets needed so you must update Adrenaline before using it on these firmwares (you can only use it on hacked FW 3.65 right now)
Interestingly enough, Adrenaline-6.1 doesn’t seem to include some commits merged into the main branch after the release of Adrenaline-6. For example, the ability to shut down your Vita from the Adrenaline menu can be found in the main code branch but it isn’t included in the Adrenaline-6.1 release. If you’re on FW 3.60, Adrenaline-6.1 will work just fine but there’s no real

You’ll have to reinstall the PSP firmware files even if you had installed them before!