A "Direct Injection" (DI) XLR cable is merely a normal XLR cable used in conjunction with a DI box. XLR stands for "Cannon X-series connector with Latch and Rubber." XLR is the standard for digital and anolog pro-audio balanced line connectivity between audio production equpment. XLR cables are 3 pin and therefore grounded and shielded against interference and noise.
An RCA cable is a cable used with consumer and factory electronics. It is not a pro-audio cable, which means that it's not balanced and grounded. Therefore it is best to keep these cable lengths short to minimize interference.
A DI box is used in conjunction with certain musical instruments, such as electric bass or electric guitar. It is used to interface the instrument's electrical signal with balanced inputs of the mixer. This device is needed because the instrument's outputs are unbalanced. The input to the DI box are generally "Tip Ring Sleeve," or TRS types, whereas the output to the mixer is XLR.