The important passages:
The Jeph Loeb/Tim Sale LONG HALLOWEEN/DARK VICTORY opus wasn't tied to mainstream continuity, allowing them the leeway to kill Year One-era characters (like Lieutenant Flass, who survived into the present in the core series' wedding of Jim Gordon and Sarah Essen) and portray a romantic relationship between Selina Kyle and Bruce Wayne that post-Crisis continuity had prohibited. The revised history had declared that Catwoman had never been captured and that Selina had never met Bruce or Alfred (BATMAN #499 and others). That said, the aforementioned Ed Brubaker has included the Bruce-Selina romance as canon, anyway (in places like BATMAN #600, CATWOMAN (current) #10 and CATWOMAN SECRET FILES #1).
That same issue of SECRET FILES also officially banished a Catwoman story (ACTION COMICS WEEKLY #s 611-614) that has long been regarded as non-canon. The episode had killed off Selina's pal, Holly (alive and well in the current series), but the detail that threw everyone into an uproar was a scene in which Catwoman threw two security guards from a skyscraper window to their deaths in order to frame Holly's killer for the murders. Even setting aside Catwoman's aversion to killing, the sequence didn't make a whole lot of sense.
Batman: Year One indicated that the pre-Catwoman Selina Kyle had been a prostitute, a development that seemed a bit too adult for a character still being marketed to children. Perhaps in acknowledgment of this, DC has shied away from that characterization since Zero Hour. 1994's CATWOMAN #0, for instance, established that the young Selina had become wealthy as a cat burglar and that the hooker persona had been a subsequent ruse to separate potential customers from their cash the moment they were alone.