Hydrogen is a drum machine for your computer with all the features of a traditional drum machine and probably quite a few more.
If you have used classic hardware drum machines, or any software based on that model, you will have no trouble settling into Hydrogen.
Even if you have never used a drum machine, Hydrogen's design is simple and intuitive, but the application itself is powerful and robust.
Hydrogen is a MIDI instrument, so you can use it with other musical tools via JACK.
Hydrogen is a drum machine, not a general-purpose sequencer, and certainly not a DAW. If you like the way Hydrogen works and want it for general music-making, see LMMS.
Hydrogen is intuitive whether or not you have ever used a hardware drum machine. The setup is common for software drum sequencing. Starting from the bottom, the drum sequencer itself is located in the bottom left. It is a “piano roll” style sequencer with the names of the drums in the current kit listed vertically, and each beat in a measure on the horizontal axis. Whatever you sequence in this panel becomes a pattern. You can have, for all practical purposes, as many patterns as you like.
On the bottom right are instrument controls and your kit library.
Across the top is the pattern editor, where you sequence which drum pattern is played throughout the course of your song.
Hydrogen drumkits are tar archives containing drum samples and a descriptive XML document. They are relatively trivial to make. The Hydrogen website hosts extra drum kits. In addition to those, Slackermedia has created and released 99 drum kits for free. The script written by Slackermedia to create kits is open source and is available at https://gitlab.com/genhydro/genhydro.
Install new drum kits in the Instruments menu → Import Library.