".OB" after a stock ticker symbol
A: Over the counter
This means the stock is traded "over the counter", which isn't actually what it sounds like. It's just traded less often, so it's not traded on the main Nasdaq or NYSE trading "floors." (Note that Nasdaq's floor is entirely electronic - there are no people shouting at each other). This is usually because it doesn't qualify for full listing on Nasdaq itself.
There are actually two related categories here. .OB means OTCBB (Over the Counter/Bulletin Board). They're still traded electronically, they're just very thin and you can't always get good prices. .PK means Pink Sheets, which MAY be traded electronically, but are also often traded by calling around to various brokers for quotes.
Both types are very thin on volume, normally, so it may be hard to find a buyer for shares you own at a price you're happy to sell at. They can also be very volatile - because there are few owners, it's very easy to push the stock price around by just buying or selling aggressively. These stocks are thus common targets for scam e-mail messages claiming some stock will jump soon - those are almost always Pink Sheets or OTCBB stocks that are easy to manipulate.
Scammers will usually do one of two things. First, either they'll push the stock up a little to convince you to buy, then madly sell to you when YOU buy until they're "jacked" short. This invariably drives the stock down hard, at which point you're in trouble and they're made a tidy profit. Or they buy slowly BEFORE they send out the scam, and sell after the price rises when YOU buy. Both are illegal, although can be hard to prove. Note that these stocks often have very low prices - $0.06 is not an uncommon price, and some are even quoted to tenths or hundredths. That means with very little money invested, especially in a prop account (if the day trading firm allows trading pink sheets) you can pick up millions of shares, and make big profits on even a 2-cent rise in price.
Other designators usually mean other markets:
.AX - Australia
.L - London
.PA - Paris
.TO - Toronto
.ST - Stockholm
etc.
Note that these particular suffixes are not universal. The ones I named above are those you'll commonly see on Yahoo! Finance. Reuters, Bloomberg, and other venues often use their own formats (such as .UK, .LSE, or .GB for London). .OB and .PK are pretty standard, though
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