The burglars, late known collectively as CREEP (Committee to Reelect the President), broke into the Democrat offices at Watergate to wiretap the offices and look for top secret documents. It doesn't appear that they were looking for anything in particular. They just wanted something that could give them an advantage over their opponents during the forthcoming election campaign. It is worth remembering in that particular election, the country had been deeply divided by the war in Vietnam and Nixon was in means guaranteed of a second term.
Nixon was eventually reelected in a landslide victory, but his campaign was marred by accusations of foul play. A month after the burglars had entered Watergate they had broken in again to redo the wiretaps and were caught. Though Nixon denied any wrongdoing, it soon became apparent he had paid the burglars hush money. Nixon's involvement was proved beyond all doubt, two years after his reelection, when he was forced to hand over the tapes that had him talking about and organizing the burglaries.
While Nixon was pardoned by Gerald Ford, many of Nixon's aides went to federal prison for their involvement.