1.hundred用作名词时的基本含意是“一个百”,如果是两个百以上,则须用复数形式。
2.hundreds的基本含意是“几个一百”或“几百(个,人,元…)”,后接可数名词的复数形式或集合名词表示“数以百计的”或“许多的”。

How many hundreds are there in thousand?一千有多少个一百?
The cake was decorated with a large hundred.这块蛋糕上面装饰着一个很大的“百”字样。
There are hundreds who need new housing.有许多人需要新的住房。
His coat is worth hundreds of dollars.他的大衣值好几百美元。
Hundreds of people were killed in the earthquake.那次地震中有数百人死亡。

［词源］＜中古英语hundred(一百)

1.hundred用作基数词表示“一百”时,其前既可用a也可用one,但用a比较普遍,在美式英语中经常省去a或one。但hundred用作修饰词表示“很多”时,前面一般用a。
2.hundred前还可加two〔three〕...等表示数量意义的数词。

There were one hundred people in the room.房间里有一百人。
I could give you a hundred reasons for not going.我可以给你说出一百个不去的理由。
He has two hundred books.他有二百本书。

下面各组中两个短语意思不同:
four hundred 400
four hundreds 4个100
some hundred 100左右
some hundreds 几百
下面两个短语都有“几百人”的意思:
several hundred of people
several hundreds of people

hundred还可用作代词,含意是“一百(个、人、岁…)”。例如:
There are about three hundred in the hall.大厅里有近三百人。
She's a hundred today.她今天一百岁了。