Fastboot : Fastboot is a special diagnostic and engineering protocol. You can boot your HTC Android Devices into fastboot mode. While in fastboot, you can modify the file system images from a computer like you can flash kernel as well as other partitinos.
ADB : ADB stands for Android Debug Bridge. Its a tool to send commnads to Android Phones from Computer Command Line.
Download Fastboot & ADB tool and driver Dev-Host - Fastboot+ADB(20926).zip
If you are more advanced and want more on android developing you can download Android SDK
Android SDK | Android Developers