What's the diff between 12AX7 and 12AX7A tubes?

Can someone tell me the difference between these tubes? I see many tubes with the "A" added to the end. Does this mean that it is a different tube? Seems to me that I have seen people interchange these as they chose. Would like to hear from anyone with some knowledge of this. Thanks,
check out Joe's Tube Lore at AudioAsylum for a detailed discussion of the various 12AX7 tubes. Also http://www.machmat.com/sales/Ecc82