ProTags are over 370 unique images that you can assign to your online persona. There is a ProTag associated with each highlight, honor, and Career milestone you can earn. To obtain one you have to accomplish certain objectives (for example, to earn a "Sacker" ProTag, you must "Get 15 Kills in a Crossfire Match"). It's a great way to show off to everyone what you have done. Some ProTags may be earned while playing a Blitz, but most require the player to participate in Crossfire matches.
Every Monday Night Combat player wears a ProTag; ProTags are seen in game lobbies and in Crossfire matches by any opposing Pro you kill—or who kills you. If you do not choose a ProTag for yourself, you will default to the "MNC" tag given to all players when they start the game for the first time.
Many ProTags can be worn immediately once earned (for example, all Honors). Others need to be "unlocked" with a one-time payment from your available lifetime earnings.