Sailfish is a Linux-based mobile operating system developed by Jolla in cooperation with the Mer project and supported by the Sailfish Alliance. It is to be used in upcoming smartphones by Jolla and other licencees Although it is primarily targeted at mobile phones, it is also intended to support other categories of devices.

Compatibility

Jolla declared Sailfish OS compatible with Android phone and tablets hardware, using existing Android device drivers. This was announced on September 2013 and it is a major milestone. This has been achieved with use of Alien Dalvik by Myriad Group. This is a big step forward for phone and tablet manufacturers which can immediately and effortlessly integrate their existing devices with the Sailfish OS and still have ability to use Android software. Unveiled in 2013 Jolla prototypes and Sailfish running on other non-Jolla devices have shown proves of compatibility and Android applications running without problems, with native speed, without lags or jams. Jolla has previously stated that Sailfish is compatible with Android applications. This was done in collaboration with the creators of Alien Dalvik, the Myriad Group, known for running Android apps with the Nokia N9 (but the Android layer was never released as built-in on N9). Many Android applications run on Jolla devices unchanged. To take advantage of all UI and other features of Sailfish OS and make applications fast they may need porting to native Qt/QML, there are extensive guides available on porting to Qt/QML.
Jolla has reported that Ubuntu, Sailfish and Plasma Active cooperation for sharing common APIs is in progress and - upon success - will make the platforms compatible on the API level.
As Jolla Sailfish is based on the open-source Meego, it is compatible to some extent with MeeGo Harmattan released by Nokia with the N9 phone. The applications written in Qt for N9 might need a minor conversion process before they can run on Sailfish.

Software availability

Sailfish will be able to run most applications that were originally developed for MeeGo and Android, in addition to native Sailfish applications. This will give it a large catalogue of available apps on launch.
Considering upon Jolla's declarations that Sailfish OS is be able to use software from following platforms

• Sailfish (natively created + ported like from Qt, Symbian, MeeGo - developers have reported that porting a Qt written software with Sailfish SDK takes a few hours only)
• Android applications are directly running in Sailfish OS. They are compatible as they are in third-party Android stores, with no needed modification (in most of the cases).
• MeeGo (because of backward compatibility thanks to MeeGo code legacy included in the Mer core)
• Unix and Linux (as Sailfish is Linux then using such a software is possible, especially RPM packages, either in terminal/console mode or with limitations implying from using Sailfish UI, if not ported and adjusted)
• HTML5 (because it is ready for this kind of software then use oncoming FirefoxOS software can also be possible)

Then a number of unique software possible to use can be estimated by adding number of software from the list above. In general it can be expected to be not less than 500K unique software pieces at the beginning.
The best thing about Sailfish OS 
You can run Android apps on Sailfish OS directly