SlideShare is a web tool that allows users to upload and share presentations in various formats like PowerPoint and PDF. It enables collaborative work where presentations can be created, edited, and reposted by groups online similar to Google Docs. In the classroom, teachers can use SlideShare to share lesson plans with other teachers and students can work on group projects together through the site. Professionals can also find and download presentations to edit or republish, helping to streamline work without having to start from scratch.