Monopoly is one of the best-selling board games in the world, and it comes in a variety of different editions.  The most luxureous edition is widely considered to be the Franklin Mint Monopoly game, produced since 1991 by the Franklin Mint (though production and availability of new sets has been limited since 2005).  Over this multi-year run, there were two similar but distinct types which were created, featuring different printing on the money, deed cards, chance and community chest cards.  The first and earlier type of this game (Type 1) included a copyright notice, "1991 Parker Brothers" on the chance cards, community chest cards, and money (the deed cards used the 1935 date in their copyright notice rather than the 1991 date), while the later second type (Type 2) also includes the text "Printed in U.S.A."  Note that there are also two different subtypes of Type 2 currency -- differentiated by whether the symbol in the lower-right corner of the "MONOPOLY" text on the reverse has a "TM" or "®" designation (this symbol is located above and to the right of the copyright and "Printed in U.S.A." texts). 

The following pictures demonstrate the difference between these two types on a the back of a $500 bill from these two sets (the top example is from a Type 1 set, the bottom is from a Type 2 set).  Notice the number of lines of printing on the lower-right corner of the reverse of the bills:

Another difference between these two types of bills is the coloration.  The first type of bills are a little brighter in color than those of the second type (this can be best noticed on the $5 and $500 bill) -- note the subtle color difference between these two bills of the same denomination but from different sets. 

Chance and Community Chest cards also have different printing as shown in this picture, and again have a slight coloration difference between sets (though not as pronounced as with the currency).

And the deed cards as noted have the 1935 date rather than the 1991 date on Type 1 and Type 2 examples:

While the differences between these bills is minor and either set would work as a replacement, if you are missing some pieces to your set and are looking for precise replacements, it is handy to check with a seller to determine whether the money or cards they have are Type 1 or Type 2 specimins.