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In VERY general terms, more cavities = more isolation. Larger cavities generally have a better efficiency - ie: less loss through the duplexer for equivalent isolation achieved. As Berkinet mentioned, the higher quality duplexers involve a combination of bandpass and reject technology. Lower cost "notch" style duplexers are usually limited to 65 to 75 dB of isolation between transmit & receive. Notch duplexers suffer in high power use, and they won't filter out other nearby transmitters very well (They are designed to only "notch out" the transmit frequency of the machine they're used on). Good duplexers have high isolation, low loss, low noise, and can handle high power. They'll also use quality components, and will be built to handle large swings in temperature without de-tuning.

Well, since it is usually better to learn how to fish than to have to buy fish... Try starting with the Wikipedia. Another excellent source for repeater information is Repeater-Builder.com. Here is a page from them on duplexers. But, to get you started... very basically, each cavity performs one of two functions. It either: Filters out everything except a specific frequency (a notch filter)Filters out just a specific frequency (a band-pass filter)As you add more cavities (I.e. the more times you filter the signal) the effect of the filter becomes more pronounced. The low cost, so-called mobile, duplexers consist of only notch filters - 3 for receive, 3 for transmit. The transmit cavities (attempt to) filter out any signal other than the transmit frequency. The receive cavities do the opposite, filtering out everything except the receive frequency. Adding a band-pass filter will boost the effectiveness of the duplexer. For example, a receive band-pass cavity would be turned to attenuate the transmit frequency. Note that most high-end and commercial duplexers consist of both types of cavities.

Ahh, I knew there was a reason. I was afraid there was a logical reason. I guess i'll have to go back to when I was a kid. Two tin cans, two buttons and a tight string.