Western Digital (WD) offer different series of hard drives, each one suited for a specific purpose.  WD has colored their drives based on the function each one is optimized for. Currently there are 6 colors, each one representing a product that's made to do something special.

The Table Below serves as a quick reckoner for different WD Hard Drive families and what each one is optimized to Do.

Family Purpose Description
WD Gold / WD Ultrastar Servers, Enterprise applications, Data Center Gold are the Enterprise Drives (AKA Server Class or Data Center Class drives ) . Highest Reliability , 5 Years Limited Warranty and obviously most expensive.  Gold is available only in SATA interface but Ultrastar is available in SATA and SAS.  Gold and Ultrastar can be used anywhere you can afford them - Servers, Data Centers, NAS, SAN, Surveillance.....
WD Red/Red Pro Network Attached storage Red drives are meant for Network Attached Storage upto 8 Bays. Red Pro are meant for network Attached Storage upto 24 bays. WD Red is available in 2.5" and 3.5" SATA formats.
WD Purple Surveillance, Video, Smart Video Analytics Purple Drives are Purpose Built for Smart Surveillance.  They work in DVRs and NVRs . Higher capacities also support Deep analytics by allowing access to recorded video while recording. Available in 3.5" SATA formats.
WD Black Gaming, performance Black offer performance for High Performance PCs, Gaming and graphics designing workstations where a higher performance is required. Available in 2.5" and 3.5" SATA drives.
WD Blue Everyday Desktop Blue Drives are for Everyday Computing. Typically used in desktops and laptops. Available in 2.5" and 3.5" SATA