there was a pretty good explanation in The Guardian yesterday.
Basically, most banks are reasonably ok, but part of the problem is that banks don't hold enough cash reserves to meet all their obligations (i.e. fill up cash machines, pay out over counter transactions, meet all transfers out of their accounts etc) all the time.
So banks lend money to each other constantly to cope with the ups and downs of the demands on their funds, if that makes sense. Bank A will borrow £1m from Bank B because that week loads of people have taken cash out of its cashpoints, for example.
The trouble is, with the credit crunch and irresponsible lending etc, a few banks have overextended themselves and ended up not being in a position to pay other banks back.
As well as making the banks go under (Icesave, Lehmans etc), this means banks are now hardly lending to each other at all because they don't know if they'll get their money back, they don't know who might go under next. And the rate of interest that they lend to each other at has gone through the roof.
So a couple of banks going under means that the whole system of money flowing round the system has ground to a halt, and all banks are now at risk of not being able to meet their obligations as they can't borrow in the normal way.
Did that make any sense?