Tutorials are any resources you learn from.
Examples: an intro to html5 screencast, a pdf about git, photoshop effects tutorials, meta-programming in ruby, lambda calculus, higher-order fixed-point combinators.
Tools are websites, apps or services used -on- your project (indirectly), to aid the process.
Examples: A color scheme generator, email marketing software, usability heat maps, css3 code generators, a downloadable png compressor.
Assets are downloadable files used -in- your projects, usually as code, textures, or images.
Examples: a jquery sticky menu, photoshop brushes, background textures, mvc frameworks, twitter bootstrap, 960 grid system.