Matt gave a good explanation. I'd just like to add a few things. With a cell array, you refer to a cell with a number (the index), with a structure you refer to a "member" with its name. For example ca{1} = 'Hello manoj' while myStruct.myString = 'Hello manoj'. You see, the number 1 for the cell index, and "myString" for the structure member (or "field") name.
A cell in a cell array is referred to with parentheses, while the contents of the cell are referred to with braces. For example ca(1) would be a cell and that cell might have a string in it, while ca{1} would be the contents of ca(1) which would be the actual string itself, say, 'Hello manoj'.
There is also a function called celldisp() to display the cell contents.