Straight cables are used to connect a DTE with a DCE device.
They have one male plug to connect the DCE and a female plug for the DTE side.
The most simple cable requires 3 wires only:
The blue plugs are female types (back view)
Cross Cables (Null Modem)
Cross cables are used to connected two DTE devices. Null modem in particular is a communication method to directly connect two DTEs.
In exceptional cases such cables can be used to connect two DCE devices, provided that the female plugs are replaced by male types.
Like with straight cables, the most simple one requires 3 wires only:
The blue plug is female type and the gery one is male (back view)