Digital differs from analogue by the amount of information that can be held in a single communication line (wire). Digital means the wire will be either high or low (0v or 5v) and nothing inbetween. Analogue means that the value can be anything between high and low inclusively (between and including 0v to 5v).
For our outputs, we have digital outputs. Because they are digital they can be either on (high which is 5v) or off (low which is 0v). The digital outputs cannot be "sorta" on, or "kinda" off. They are either on or off no ifs, ands, or buts.
For our sensor inputs, we use a single wire per sensor to transmit values to the Fusion Brain. Because it is not useful to know if the temperature is on or off, but rather know what temperature it is, we must connect it to an analogue input. The sensors will scale a value between 0v and 5v that the Fusion Brain will understand and transmit to the PC.
Because 0v and 5v are included in the analogue input ranges, you can connect a digital input source to an analogue input channel on the Fusion Brain. Oppositely, because the range of analogue values is everything inbetween 0v and 5v, you cannot connect an analogue input source to a digital input channel on the Fusion Brain.