74HCT373 Octal transparent D-type latches
74HCT373 - High Speed CMOS Logic Octal Transparent Latches with 3-State Outputs. The 'HCT373 devices are octal transparent D-type latches. When the latch-enable (LE) input is high, the Q outputs follow the data (D) inputs. When LE is low, the Q outputs are latched at the logic levels of the D inputs.
What it can do ? - This is like a Byte of Memory, A Byte means 8 Bits. A Bit Means one Input or Output Pin of an IC, Which understands Logic 1 is High and Logic 0 is Low.
The HCT works well with old TTL and CMOS families. This takes very less Power (battery lasts longer). It switches very fast (the computation is done faster). Its got good symmetrical output drive. Means Source and Sink both nice.
When you run out of ports I/O in your favorite uC, This guy will come to your rescue. Simple way is to Put four of this and Interconnect or Bus their Data inputs D0-D7. Then Connect this Bus to 8 Ports of our uC Chip. Enable the Output of all of them. Use 4 ports from your uC to latch each one with the data you want to be outputted. Now you have 8x4 32 Bits output. This comes at a small cost, the clock time, but negligible. In the same way you can enhance the Input ports, use the 373, the other way around with OE.
74HC373 and 74HCT373 - 74hc373-74hct373.pdf
This all means more Keys, Sensors, Motors, Relays can be accommodated. By Multiplexing Matrix even more can be achieved. You can also expand Analog Inputs using Analog Multiplexers. Try CD4051 or even FETs.