Like any other hobby, coin collecting has its own jargon in order to avoid misunderstandings when collectors talk about coins. "Obverse" is what is considered the front of the coin, and "reverse" is what is considered the back of the coin. They use these terms rather than front & back or heads & tails since obverse & reverse have standard, agreed-upon definitions, where the other terms can lead to confusion.
For U.S. coins, whichever side has the date on it is considered the obverse (with a couple of exceptions).
I asked this same question in the Usenet group rec.collecting.coins, and got this response for world coins:
"For world coins, the rule followed by the Krause Standard Catalog of World Coins is that for countries with royalty, the bust is the obverse.
For republics and other non-royally ruled countries, the side with the name of the country is the obverse, so neither the date nor the denomination
enter into it."
Alan Herbert - Contributing Editor - World Coin News