Jingle calls are a kind of message used within XMPP, kind of like the noise your modem makes and a spoken sentence are different kinds of "messages" the telephone system can handle. Resources are more analogous to extensions on a telephone system. You can log into the same account on the XMPP network multiple times, as long as the resource name each client is using is unique. This resource identifies which client currently signed in you mean. For example, my single Jabber ID (xmpp:ba....@ursine.ca) is usually on XMPP times three:

1) The login I'm sitting right now is on ursa- major.network.ursine.ca. Since I use Psi, I can tell it to use the hostname as my resource name (since it's generally safe to assume that two different client systems used by a single person are not going to be named the same thing). This resource is thus uniquely identifable at xmpp:ba....@ursine.ca/ursa-major

2) I use different resources from gmail (so I can see statuses while in email...it's too bad gmail's webpage doesn't have an invisible mode for that, though I guess for that to be useful, Google's clients would have to *not* all take advantage of the fact Google, in all it's wisdom, can't seem to comprehend a sane way to handle resources between different gmail/gtalk logins) and in my delivery truck (so people who need to get a hold of me immediately and know to message that specific resource can do so when it passes near a wifi zone), etc. at lower priority values. They appear as xmpp:ba....@ursine.ca/ gmail<insert garbage here>, etc.