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"BPO is a monopoly on an in-game item. (Well, there's 12-20 BluePrint Originals for each rare item). You normally win them in the in-game lottery, and it's the gift that keeps on giving. For WoW players, it'd be like purples not dropping and only being available from 1 player crafter per shard."

Not quite. To clarify to others: BPO means "Blueprint Original". To produce any item in EVE, you need a blueprint and raw materials. Originals may be used for production, may be researched to reduce material costs and production time to a certain degree (the limit being the maximum reprocessing value of that item. i.e. if a maxed out refiner reprocesses an item that was produced with no material wastage due to good research and producer skills, no materials are lost.) Last, BPOs can be copied.

Note that copies cannot be researched, cannot be copied again, and are limited to a certain number of production runs before they go poof. At one point copies could be used for unlimited production, and this killed the economy for at least a year and a half. While CCP made all new copies limited-run within a matter of a few months, the unlims were left ingame for at least another year before they were all converted to maximum-run-count copies.

In the game, there are currently three main classes of items:
Tech 1 - The baseline. The Tech 1 (referred to usually as T1) variant of any item is typically the worst variant (except for some items which have "Civilian or Basic" variants) T1 BPOs are one of the few things that can be bought off of the NPC market. There is an effectively infinite supply, so any player with enough money to invest in a T1 BPO can start producing it. For a while there was almost no money to be made in T1 unless you were first on the market (see the above comment about unlimited copies, from here on referred to as BPCs) Now that all BPCs are limited-run, there is now money to be made from T1 items, although not huge amounts.

Named items - All better than T1 variants, with increased effectiveness and reduced fitting requirements. They are NPC drops only, and no BPOs exist. Occasionally BPCs for these may be given out in various manners.

Tech 2 (T2) - T2 items have improved effectiveness at the cost of increased fitting requirements. (The exception being ships - they are just plain Better, and also are designed for specialization.) Some named items are better than T2 variants, although this is rare. T2 items can only be produced from BPOs that are essentially given out in a lottery, where lottery "tickets" are purchased by running missions for special NPCs in the game to gain what are called research points. There are only approximately 20 BPOs for any given T2 item in existence in the game. Additional BPOs are only given out if a BPO owner gets banned or CCP finds that they have gone inactive or have left the game. (i.e. the BPO isn't being used.) Due to the limited supply, T2 BPOs mean massive profits even though the material costs can be quite high. The thread linked to is talking about potentially buying out all (or most) BPOs of a given item, establishing a full monopoly on that item. You may see references in the thread to HACs - That stands for Heavy Assault Cruiser, and that particular type of ship is increasing in popularity but supply is not changing, so HACs are climbing steadily in price (and profit for the BPO owners).