Well,
¥ is the international symbol for the Japanese currency (usually read 'Yen' or sometimes still 'Sen', while
円 is the tradional Japanese Kanji for the Japanese currency (and is read "en").
Accordingly ¥ is placed in front of the sum whereas 円 is placed behind the sum of money. You can see this if you take a look at the backside of any Japanese book: Below the two barcodes you will always find something reading like this:
ISBN4-10-353413-3
C0093 ¥1600E
定価: 本体1600円(税別)
The first line after the ISBN shows the information about the books price in international notation standard (1600 Yen - the "E" meaning "tax not included") whereas the third line holds the the same info in Japanese notation.
The Kanji 円 also bears the additional meaning of "circle" or "round".
Tradionally a "sen", written 銭 was worth a 1/100th of an "en", but it is no longer used nowaday since the smalles coin in Japan has the value of 1 Yen (or "en"). Afaik, in some countries the word "sen" was used as the official name for the Japanese currency until about a decade ago or so, but personally I haven't seen it used anymore recently.
Hope this helps?
ja ne,
-andy