Firstly - Davinci Resolve (since v15) now includes Fusion as a sub-tool. Additionally some component features of Fusion are embedded within the Resolve interface and conversely some features of Resolve are accessible from within the Fusion tool.
So ... I will try and answer below - as if Resolve did not include any Fusion components:
Resolve has its well established roots in Color Grading, but both applications are ultimately editing and compositing tools which can generate, combine and manipulate source material to create desired visual & audio output. Resolve is targeted at Ease of Use, Media handling, Video Editing, Compositing, Color Grading, Audio Engineering, Audio/Video Effects and Content Delivery. Whilst Fusion is targeted at advanced 2D & 3D Compositing, Visual effects and Motion Graphics.
Fusion is a more complicated tool and typically is slower to work with, but provides advanced functionality, precision, programming and control in the areas of VFX and MG. There are many end-to-end Audio/Video production scenarios where Fusion would not be required.
Typical Use Cases:
Resolve (minus Fusion):
- Producing simple videos from source material
- Editing complex video sequences
- Color Grading / Color Correction (Simple -> Advanced)
- Audio Engineering
- Transcoding & Final Delivery
- Applying Audio/Video effects to existing footage
- Creating/Applying simple Computer-Generated Imagery (CGI)
- Generating/Applying advanced 2D and 3D Video Effects (VFX)
- Creating/Applying Motion Graphics (Animated Titles and Wurly Spinny things)
Fortunately - Resolve now includes Fusion - so you get the best of both worlds integrated in one hugely powerful application package.
And yes - you can complete an entire Film 'Movie/Short Film/Video' using just Davinci Resolve and Fusion tools.
** Fusion is also available as a standalone product, but the features & functionality of the standalone product are almost entirely available within Davinci Resolve.