When connecting to an AV system, typically there are two methods.
The first is VGA, which has been commonly used for computer/laptop connections for a long time. This is an analogue-type connection and requires the addition of a 3.5mm audio connection to reproduce the sound from a device. So, the (generally) blue connector gives you the visual, whilst the 3.5mm jack provides the audio.
VGA was the main connection for many years, but now a digital version called HDMI is becoming the standard for connection to projectors and monitors. HDMI is a more reliable, higher resolution (up to 1920×1080) connection. It also includes the Audio component in the cable, so there is no need to connect a second cable (like the 3.5mm cable with VGA).
So, if you have the choice, always go for the HDMI connection for reliability and a higher screen resolution (clearer / sharper images).
Then, more recently, projectors and LCD TVs started to come with interactive capabilities. This is where the USB connectivity comes in. If you don’t have USB connected to both laptop & projector, there the interactive component of the projector will not work.
Please watch the video below for a quick overview of these connections.