Apple's Radar system is also open to registered developers who can use it to file bugs or enhancement requests with Apple. Although Radar is reputedly very good when used inside of Apple, this is not generally apparent to users outside of the company. The frequent request to "file a radar" often sounds more or less like "forget it, buddy", because issues frequently go weeks or even years without so much as an acknowledgement or indeed any indication at all that anyone has even read the issue. This leads to the perception of the Radar system as being something of a black hole.
External users have no way to know if an issue has been reported, so it's common to file an issue and then be informed that it's a duplicate of a previous issue. That seems reasonable at first, but in practice it means that you'll never get notified if the issue is resolved.
The desktop app mentioned in another answer here is not available to external users, so it's purely a web site.
Edit: as an example of the issues external users face with Radar, yesterday a number of developers had issues closed with a message reading:
We are closing this bug since our engineers are aware of the issue and will continue to track it.
Which essentially says that the bug has not been resolved but that it's going to be closed anyway, and good luck to the reporter if they ever want to find out if any change has been made. These bugs might be fixed someday but no effort will be made to notify the people who reported them in the first place.
Radar is Apple's bug tracking system. It's developed in-house for use at Apple exclusively, and it tracks absolutely everything, from bugs to tasks to enhancements to features to problems with content to issues with IT, and anything else you can think of.
It supports many different workflows for different teams, it's fairly easy to use, it even includes detailed security settings so you can't see radars on components you don't have access to, it keeps the entire history of everything that's happened, it even lets you 'CC' yourself on a radar so you get reports when things happen to it (so you can see the progress of radars you're interested in), and lots more. It's also managed via a native desktop app instead of a website.
It is a software bug tracking system somehow similar to the IBM's UTP (unified tracking platform). You can submit a ticket for whatever problem you have and hope it will be solved. The ticket then gets from state submitted to assigned, commited, resolved, etc, (depending on the workflow), At the end it gets solved, rejected, marked as duplicate, deferred, etc etc.