Co-browsing is browser-based, meaning that you only share the browser instead of sharing your entire screen (which is the case with screen sharing).
Screen-sharing solutions are pixel-based, the drawback of this approach being that screen updates are slow and of low quality. Most screen-sharing tools require external software to be installed by users. This makes it unsuitable for most web situations, as people are often unwilling to install extra software that circumvents the browser's security measures.
Further, screen-sharing opens you up to chances of unwanted private information leak. On the other side, with Surfly Cobrowsing you can even mask specific parts of the web content you want to share with others.