A straight-through serial cable is the most common type of serial cable, used to connect a Data Terminal Equipment (DTE) device (for example, a computer) to a Data Communications Equipment (DCE) device (for example, a modem). The pinout of the serial connectors on both sides of the cable are exactly the same with a straight-through serial cable.
A null modem serial cable (frequently called a crossover cable) is used to connect two DTE devices together without the use of a DCE device in between. For this to happen, the Transmit (TXD) and Receive (RXD) pins on one of the serial connectors are flipped.
To confirm the type of serial cable that is required for your device or configuration, refer to the information provided by the manufacturer.