A DAW (Digital Audio Workstation) is the software that turns your computer into a recording studio. It records audio from your interface, hosts virtual instruments (VSTs, AUs), provides mixing tools (EQ, compression, reverb), and exports finished tracks. Popular DAWs include Ableton Live (electronic music, live performance), Logic Pro (Mac-only, all genres), Pro Tools (industry standard for recording/mixing), FL Studio (beat-making), and Reaper (affordable, highly customizable).
Audio interfaces almost always include a 'Lite' or 'Intro' version of a DAW. The Focusrite Scarlett series includes Ableton Live Lite — a fully functional but track-limited version. PreSonus includes Studio One Artist — a full-featured DAW, which is the best bundled software in the industry. If you do not already own a DAW, factor the included software into your interface purchase decision.
DAWs share the same core functionality. The differences are in workflow (Ableton's session view vs Logic's linear timeline), included instruments/effects, and ecosystem (Pro Tools' industry compatibility, Logic's tight Mac integration). Most producers try a few before committing — Reaper offers a generous 60-day evaluation period.