There are four caravans that you can encounter while exploring the wasteland:

• Crazy Wolfgang's caravan sells miscellaneous items, plus Schematics - Rock-It Launcher.
  
  
• Crow's caravan sells armor.
  
  
• Doc Hoff's caravan sells food and meds, plus Schematics - Nuka-Grenade.
  
  
• Lucky Harith's caravan sells weapons, plus Schematics - Shishkebab.

The caravans follow a fixed route that takes them through cities like Arefu, Canterbury Commons, Megaton, Paradise Falls, and Rivet City. If you wait at a caravan stop for 1-3 hours, then the current caravan (if any) will leave, and a new caravan will show up, and so you'll quickly be able to visit all of the caravans if you like.

When you travel to Canterbury Commons, you'll be able to convince Uncle Roe there to organize the caravans, and then you'll be able to invest in them. Investing in a caravan will increase its stock, its security force, and the amount of caps it carries. If you invest in a caravan twice (for 700 caps total), then you'll also receive a reward: Crazy Wolfgang will give you some Stealth Boys, Crow will give you Crow's Eyebot Helmet, Doc Hoff will give you some Stimpaks, and Lucky Harith will give you a Mini Nuke. If you have the Master Trader perk, then the investments will only cost half as much.

Note: Sometimes the caravan traders can get killed, and even if you resurrect them (using console commands on the PC), it's possible that Uncle Roe won't recognize that they're alive, and so you won't be able to invest in them.