Upwork is a platform where you can offer services similar to Fiverr. Upwork tends to be more focused on freelancing and freelance services, whereas Fiverr tends to be on a variety of services.
Upwork is project-based, and Fiverr is gig-based. Upwork’s system for securing jobs is through what they call “connects.” On Fiverr, you list your gig, and consumers can purchase it.
When considering which platform to use, ask yourself which one you understand. Also, think about the type of things you’ll offer—you may not want to be a freelancer, and that’s mainly what Upwork is for.