It was already talked in this Q&A how to get the Developer Preview and the Technical Preview.
But what I really want to know is what's the difference between them in a user perspective.
Windows Phone Stack Exchange is a question and answer site for enthusiasts and power users of Windows Phone OS. It only takes a minute to sign up.Sign up to join this community
This is a programme that allows developers and enthusiasts to get new versions of the Windows Phone operating system as soon as they're finished, directly from Microsoft. The goal is to let users bypass the delays usually associated with manufacturer- and carrier-specific testing and approval of new OS versions, as well as to enable developers to get a head-start with app development.
While some bugs may still remain, the majority of users should experience no issues in day-to-day use. For more information, see this answer.
This refers to early, decidedly non-finished versions of Windows 10 (released now for both PCs and smartphones), with the goal of gathering as much feedback and usage data as possible before the OS is finalised.
Unlike the Developer Preview builds, you can expect to run into bugs, and there's even the possibility that you'll brick your phone. For this reason, Microsoft discourage the use of Technical Preview builds on your main (or only) phone. Regular users should stay away from this unless they have a spare phone they don't mind sacrificing.