The Recording Industry Association of America said the various suits are for "John Doe" defendants whose true identities are unknown. The RIAA has to sue them before their ISP's will tell them who they are under the threat of a court order.
This usually allows the briefs to visit the file sharers, give them a chinese burn, and attempt to get them to pay up or face the music. So far most of the file sharers have paid up when given an offer they couldn't refuse.
Meanwhile 152 people who were offered the chance to settle but ignored the RIAA briefs when they came a callin have now been sued again for huge amounts of cash.
The RIAA represents Warner, EMI, Bertelsmann, Sony, and Vivendi Universal. Its briefs have sued nearly 4,700 people including children, old ladies, and students. So far no actual commercial pirates seem to have been caught in their net. µ