A lot of it has to do w/ semantics. Back before the major 'alliances' of today, you'd frequently hear the two (or more) carriers involved in a new codeshare, announce it as their new 'alliance'. In today's parlance, I'd say pretty much what Blink182 said is correct. A codeshare is a defined set of flights where one carrier will place its code/flight number on another carriers opertion. It may or may not be reciprocal. Alliances may/may not involve code shares, but they usually do allow mileage accural across the participants, and they (at least in theory) attempt to coordinate operations/programs to provide a 'unified' experiance to the pax no matter which partner they are travelling on.
"The two most common elements in the universe are Hydrogen and stupidity"