The rule is one put forth by the Federal Reserve Board. Pattern Day Trader is someone that make 4 round trips (roundtrip = 1 opening (buy / short) and 1 closing (sell /cover) trade for the same security). That's the definition of a round trip as far as the industry majority, and the FRB.
Once you meet that, you will need to have $25,000 equity in your account, if you don't, you will get a call. In return, the FRB allows 4 to 1 intraday and 2to1 (Reg T) overnight. However, as we all know there are many firms that offer 10,20 and 30 to 1. One of the caveats that you will find from firm to firm is how the trades are journaled: FIFO or LIFO, which may contradict the PDT rule. The clearing firms and corresponding firms (B/D's) that work with them, know this rule inside and out.
In case you didn't know, the rule was a direct result of the US Senate investigation of Day Trading. This is the way the govt controls the market, with it's regulation of the mkt (SEC) and regulation of margin (FRB). They keep daytrading margin rule sin effect for everyone that has $25K, and keeps out those who don't. If you don't have the $25K for daytrading, there are firms out there who will lend the capital. Hope that helps, good luck.