In Second Life, the term "land baron" can have different meanings to different people. The largest use of the term "land baron" could be summarized as a resident who buys land at a low price, alters that land in a somewhat unpleasant way, and then sells the land for a much larger price with the goal to make a large profit.
Land barons first started appearing after SL 1.2 and when GOM and IGE became popular. With the ability to find/buy land quickly and easily, then sell for profit and cash out for USD, early land barons made an easy profit. Before SL 1.2 and L$ to USD exchange was possible, much of SL's land was sold for a low price, or even released to the public (meaning the land could be claimed by anyone for 1L$ per square meter). Therefore, residents who remembered the "good ol' days" were infuriated to learn of such business practices that land barons used. Concerns that were brought up over this were how new residents with little money could purchase land and also how any non-rich ones could as well.
Linden Lab (LL) has enacted a few "controlling forces" on the market to still maintain control so that new residents can still experience land. LL's first combatant was to bring a flurry of new sims online, somewhat flooding the market. However, they were slow to go on the auction block (the chosen method of selling land by LL), which inadvertantly put a large cork in the flood wall. Land barons quickly moved to the auction system, outbidding other residents and attempting to resell the land even higher. LL then came up with Land for the Landless, a program in-which new residents could post on a forum thread requesting land for 1L$ per sqm of variable sizes. Soon, Land for the Landless became very popular and was moved off the forums and built into the client itself.
In February 2007 LL discontinued the First Land program, citing the rampant fraud residents had noted since the programm's inception.
Currently, land barons still roam SL and cause much land issues for some residents. Using scripts such as land scanners to look for land, "alt accounts" to buy Land for the Landless and resell it, and other tactics to gain a hand in land profit. Linden Lab still releases large amounts of sims and has public land now listed on the forums to even the score a bit for other residents. However, Linden Lab also somewhat encourages land barony by often speaking to the press of how one can make a profit through the selling of land.
Land barons have become a type of "villain" within Second Life, but do also sometimes help balance out the economy in some way. Residents have also pointed out that the community itself is to blame in part for the creation of such tactics by buying into the high prices and not taking a global stance or boycott on such things. Land barony has also led in large part to the demise of public land and finding any such land priced at 0L$ or 1L$ per sqm before a land baron claims it is seem to be extremely lucky.
Land pricing is always a matter strictly between buyer and seller. Additionally, there are no public records of land transactions so the posted price on any lot is merely an offer to sell and not a record of the sell price.