In simple terms, an "A" plan is a classic pension: you receive a check every month for the rest of your life. It's usually based upon a percentage of your highest three or five years of pay multiplied by the number of years you worked for the company.
A "B" plan is a lump sum benefit that you receive the day (or shortly after) you retire. It's usually based upon a percentage of your total earnings with the airline. Also, most B funds have various options where the pilot can control how his B fund money is invested.
That's it in a nutshell -- it's obviously more complicated, and every company has their own spin on retirement, but if you think "A plan = pension" and "B plan = lump sum" then you're on the right sheet of music.

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