The only outward difference you are likely to see with new Word file formats is if you use macros or code. Previously, there were just two types of Word file types: documents and templates (.doc and .dot). With Word 2007 there are four file types: .docx, .dotx, .docm and .dotm (the "x" stands for XML and the "m" for macro).
Basic documents and templates (.docx and .dotx) can no longer contain macros or code, which is how they are made safer for everyday use — no-one can slip hidden code into a document. But because macros are useful things to have, there are two additional file types to support documents and templates which contain code: .docm and .dotm.