On Skynet, pinning doesn't mean hosting the file from your machine, it means paying a bunch of service providers to host the file for you. When you pin content to Skynet, you can turn off your computer 5 minutes later and the data will still be available globally. Just like IPFS, data is content-addressed and anyone can choose to re-pin the content.
Service providers are held to 95% uptime each, which means doing something like 10-of-30 erasure coding can get you 99.99% uptime on the file as a whole. The low uptime requirement for individual providers dramatically cuts costs and allows amateurs to be providers on the network. The erasure coding algorithms ensure data availability despite a relatively unreliable physical layer.
The other problem Skynet set out to solve is performance. In our experience with IPFS, if you aren't talking directly to a node that's pinning the content, IPFS is very slow. We've heard stories of lookups taking greater than 10 minutes for data that isn't pinned on a major gateway.
Skynet uses a point-to-point protocol rather than a DHT, which not only makes it faster, it's also more robust to abuse. DHTs are pretty famously fragile to things like DDoS and active subversion, and Skynet has been designed to be robust and high performance even when malicious actors are trying to interrupt the network.
Other than that, we've tried to make sure Skynet has feature parity to IPFS. Content-addressed data, support for building DAGs, support for running applications and static web pages, and then we've added a couple of elements of our own, such as APIs that allow applications and web pages to upload directly to Skynet from within the application.
https://siasky.net if you want to give it a try for yourself.