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Just to muddy the waters, vendors can confuse what's a L3 switch vs. what's a router. For example, a 6500 and 7600 chassis were pretty similar because they can use some of exactly the same line cards and supervisors, and they even used to run the same IOS image. Yet, the 6500 was a "L3 switch" and the 7600 was a "router".
Generally, a L3 switch has dedicated hardware for packet forwarding and is often feature poor compared to a "router". It also, as Peter describes, often supports many ports, and those ports often support L2 switch features.
Router ports often don't support L2 features, and feature support is often much more, both hard (like non-ethernet) and soft (additional routing protocols or QoS features, etc.). Do know, some "routers" will support L2 (and even L3) switch modules or ports.
BTW, when Peter notes routers can be slow compared to L3 switches, it's usually more a question of overall capacity. For example, for the same price, a switch might support 48 gig ports at wire speed while a router might only support a pair of gig ports at wire speed.
Also know, some "routers" have special hardware to increase their capacity. For example, the old 7200/7300 series with PXF, the old 7500series with VIPs, and the current ASR1k series with its QFP.
