Both platforms focus on scientific collaborative authoring: Overleaf is an online LaTeX and Rich Text collaborative editor and publishing tool and ShareLaTeX is an online LaTeX collaborative editor and reviewing tool. Their rich feature set includes: pure LaTeX editor, rich text editor (Overleaf only), real-time collaboration, real-time PDF preview, author comments, track changes (ShareLaTeX only), version control (w/ Git access) and export as PDF & LaTeX.
Overleaf and ShareLaTeX are joining forces, bringing their teams and services together as they continue to build great tools for collaborative authoring. Over the coming months, they will be working on merging Overleaf and ShareLaTeX together into a single service while making the transition as smooth as possible.
Please visit https://cern.ch/authoring