Mad Labs is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

When you encounter this phenomenon, starting with the next player in turn order, that player chooses a permanent type. The next player chooses a number between 1 and 10. The next player chooses a keyword counter. Then reveal and resolve the following ability with those words. (don't reveal the second ability before the words have been chosen!) Each player creates a token that's a copy of a [permanent] and puts [number] +1/+1 counters on it. If it's not a creature, it becomes a 0/0 creature in addition to its other types. It enters with [keyword] counter.