a=10 assigns the value "10" to the shell variable "a".
export "a=10" assigns the value ... and propagates that value to any child shells.
Use "export" whenever you want a variable to be accessible to any subshells of your script.
You can find more details here:
anomie and I apparently clicked "send" almost simultaneously (hence this edit). Lest there be an confusion - we're both saying pretty much the same thing.