A device or system which can forward IP packets at layer 3 and often posses functionality to switch packets at layer 2 as well. An IP switch uses mechanisms to classify which packets will be forwarded at layer 3 and which will be switched at layer 2 and then redirect some or all packets over a layer 2 switched path. Most IP switches utilize an ATM switching fabric, but other technologies can be used as well.
IP switches are useful as they only utilize the bandwidth needed for a packet on a single interface. This is not like a layer 1 ethernet hub or repeater switch rebroadcasts each packet out on all interfaces. IP switches are more useful than layer 2 switches because they need not rely on the underlying layer 2 protocol to switch IP. You can switch a packet from a ATM interface to a frame relay interface just as easily. IP switches are often put at the cores of large networks for these very properties.
Manufacturers of IP switches include Cisco, Juniper, and Extreme Networks. There are scores of other smaller players in this market. Many of the successful ones can switch IP packets at wire speed.
MPLS, or Multiprotocol Label Switching, is a technology which is slowly but surely creeping into this market segment. MPLS switches operate inbetween layer 2 and layer 3 to operate and are protocol independent.