The .PK is an example of a suffix representing where the security is traded - an over-the-counter (OTC) network or an international exchange. The major U.S. exchanges such as the NYSE, Nasdaq and AMEX do not have suffixes. The .PK behind a stock simply means the stock in question is traded on the pink sheets (or the Pink Sheets Electronic Quotation service).
The pink sheets service is a loosely regulated over-the-counter, decentralized market. There are few requirements to being listed on this network, as companies do not have to file with the SEC nor keep updated financial information. The only major requirement to being listed is to have at least one market maker, who must be registered with the SEC and a member of the NASD. The market maker is responsible for quoting the latest trading price of the stock on the pink sheets network. This is a highly speculative and risky place to invest. Investores would be wise to only invest what they are willing to lose.
The other suffix for an over-the-counter market is .OB. It means securities are traded on the OTCBB. This network is also one of high risk but is less risky than the pink sheets, as it is a regulated and has stricter listing requirements.
Suffixes appended to the end of a stock symbol most often represent exchanges outside of the United States. For example, Canada's Toronto Stock Exchange is represented by a .TO, so if you see a stock symbol with a .TO suffix you'll know it trades on the TSX. All major foreign exchanges have a suffix designated to them.
Here is a list of a few international exchanges: