The permaweb is Arweave's solution to a fundamental problem of the web: it's temporal transience.
The original vision of the traditional web was to enable humanity to build an accessible collection of everything known to us. While this project has been largely successful, it suffers from one critical flaw: the web has no memory. ~ What is the permaweb? Arwiki
Arweave not only allows for the storage of files, but static web pages and, crucially, fully decentralised web applications. These applications are deployed directly on Arweave and, while you navigate to them in the same way you would navigate to any website through a browser, once deployed will exist indefinitely as long as the Arweave protocol is running.
Once launched a permaweb app cannot be modified and continues to run with any ongoing maintenance. Developers and teams can continue to work on the app and deploy updates. But each update will be separate, meaning on the permaweb if you don't like the update to your favourite app you can carry on using the previous version and it will continue to work just as it always did.
Once included in the network, these files are always accessible, from any place in the world, at any time. Further, once submitted nobody (including their original uploader) can alter them in anyway -- enforcing consumer integrity for applications, and verifiability for documents, forever. ~ arweave.org