Technical Preview is what Windows 10 preview is (was) called. Preview for Developers on the other hand is the preview version of Windows Phone 8.1, which is deprecated so to say. It required a developer account - a limited one, obtainable for free through the app studio web application - to participate. The Windows 10 Tech Preview requires a Windows Insider account, which you can get for free too:
https://insider.windows.com/
Windows 10 for devices < 8" (Windows phones and small tablets) will follow the same path. Beware though, all these builds, regardless if they're for desktop or mobile devices, are beta builds. Far from being final. And they're often buggy and unstable. So if you participate, you do that on your own behalf.