There are four caravans that you can encounter while exploring the wasteland:
The caravans follow a fixed route that takes them through cities like
. 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
, Crow will give you
Crow's Eyebot Helmet
, Doc Hoff will give you some
, and Lucky Harith will give you a
. 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.