What is the difference between Megapixel and HD Camera?

HD is a type of megapixel camera.  All HD cameras are megapixel but not all megapixel cameras are HD.
While standard definition cameras (e.g., analog cameras and 4CIF IP cameras) have no more than 400,000 pixels, all megapixel cameras (including HD) have 1,000,000 or more pixels.

HD Camera Key Features Compared to Typical Megapixel Camera

• Maximum HD camera resolution is 2.1MP, maximum megapixel resolution is 16MP
• HD video format is 1280 x 720 or 1920 x 1080 (megapixel cameras can offer many more formats)
• HD aspect ratio is 16:9 (compared to 5:4 or 4:3 in other surveillance cameras)
• HDTV has quality compliance standards (where megapixel simply specifies the number of pixels)

Your typical HDTV and Blu-ray video is 2.07 megapixel

1080 vertical lines and 1920 horizontal lines of resolution or 2,073,600 pixels

Megapixel surveillance  cameras come in 1, 2, 3, 5,10 and 16 megapixels (or higher).
Anything higher than 2.07 megapixels is not HDTV standard and that means... it is not HD.

Megapixel camera resolutions: