Capture One has two main ways of working. One of them is a standard catalog, very similar in nature to Lightroom. It represents "everything you've ever done over time since you set up the catalog". For example, everything since the beginning of time or if you create a catalog per year, that sort of thing. There's a database associated with it that has all of your adjustments, previews, ratings, etc. If I want to move a catalog from one computer to another, I have to copy that database, the images, and all of the files and folders. I'd likely have to resync the image locations and other little things.
Sessions are unique to Capture One and is a concept that goes back to their days as a studio tool in the late 90s/early noughties. It represents work that you do over a short period of time. For example, a landscape shoot on a given day. Others can use them to represent a wedding or other event shoot or a product shoot. A "single thing". Unlike a catalog, everything is self contained in a folder with subfolders for images (a "capture folder") and exported JPEGs/TIFs. In addition, all of the main adjustments are stored on the file system in a human readable file and not a massive database. There is a "session database" but it really has minimal info in it. I can blow it away and essentially recreate it if needed. I can move a session folder to any computer or iPad or whatever (in the future) and pick up where I left off without moving large databases, re-referencing/syncing images and so forth. If needed, I can hand the session folder to a retoucher or an editor who can then do the work and hand it back. Later, when I've completed my edits, I can import images I've worked on into a catalog for global and more sophisticated searches.
