What is the difference betwenn "Slot A" and "Socket A" ?

I am a bit confused about the difference of motherboard sockets
labelled "Slot A" and "Socket A".

Often it is written that AMD Athlon CPUS fit into SLot A
and at other places into Socket A.

So if Athlon CPUS fit into both types then the socket types must be the same ?

Or are there any differences ?
Which one is the "better" ?

Can both socket types hold AMD Athlon mobile resp AMD Athlon XP mobile CPUs ?

Peter