Conventional CCTV cameras (Analogue) give an acceptable image for most domestic properties. However if you need super-high resolution megapixel cameras are the solution.
The best analogue cameras can achieve only 0.7 megapixels of resolution and many offer much less. A 2 megapixel camera can give 5 x the resolution of a good analogue camera. Due to good marketing many people assume that HD is the highest quality if image available. So what is the difference between High Definition and Megapixel. HD cameras are indeed megapixel cameras but HD is standard (either 1280 x 720 pixels or 1920 x 1080 pixels) some megapixel cameras can achieve much higher.
There are certain additional considerations when looking to install HD cameras. Lighting is crucial and without a good level of complimentary lighting then no useable video would be produced through hours of darkness. This is obviously an additional expense which must be factored in particularly if the cameras are sited externally.
A second factor for consideration is storage space. Recorded HD video requires a large amount of memory to ensure that recordings retain a high degree of resolution. Most HD cameras transmit via iP technology (Internet Protocol) and unlike conventional cameras do not use co-axial cable but Cat5 network cable. In essence an ip HD camera system is a networked system.
High end commercial CCTV can use megapixel cameras with resolution way in excess of HD however HD is accepted as an extremely high quality system for most homes.