FL Studio is a pattern-based sequencer. This is somewhat different than the more "normal" sequencers in that they are more arranger-centric. While being pattern-based is different, it is also very easy to understand and can lead to highly creative results.
The basic interface and workflow rely on 3 main areas that work together. The Pattern, The Playlist, and The Mixer
Pattern
The pattern is where you will generally create the various musical phrases or "loops". These loops are created by placing a new channel in the pattern and assigning it to play back something. These can be drum beats, samples or instrumentation.
A pattern channel can be any type of sound source supported by FL Studio whether it's a VST instrument, a built in generator or aWAV/OGG/MP3 sample. The Pattern supports note entry in either the step sequencer or the piano roll (FL Studio Producer and XXL only).
Each channel can be further augmented by assigning it to a Mixer channel to be further processed.
Mixer
The mixer represents your typical mixing board interface. Here you will be able to control various sounds from pattern channels that have been routed to mixer channels. The main fader controls the overall volume and there is the ability to add more effects (built-in, VST, DX) to the sounds routed to each individual channel. This makes it useful for routing groups of sounds together for processing. For example, you might route sounds of Cellos, Violins, and Violas to a single mixer channel and apply reverb to them as a group.
Playlist
This is where it all comes together. The playlist represents a series of patterns and audio clips to be played simultaneously and in a specific order. This is equivalent to arranging your song. You might have a Pattern with a drum loop play along with a pattern containing a recorded guitar part for example. You can add remove patterns from the timeline as needed to create a fully arranged musical composition.
Conclusion
This is a very basic explanation of FL Studio and how it works. For more specific guidance, browse the rest of the site or use the search function to find out more information.