I'm assuming the OP is asking which file format he/she needs to save their word processing formats in order for Windows people can read them? If this is the case then a general rule is to save the file as rich text format (.rtf) or if your documents don't have any fancy extra features such as underlining, italics, etc then you can save the file as text format (.txt).
Both of those file formats are pretty universal meaning they can be read by both Windows and Mac and also by people who don't have the same word processor as you do. It's even easier, as vansmith said, if your word processor is available on both Windows and Mac. Software such as Open Office is available for both so you don't have to save it in any special file formats in order for others to read.
If the OP is asking about other software other than word processing software then this becomes more difficult. One easy way to solve this problem is that Macs have a built in save to PDF files feature. Most Windows people by now have the free Adobe PDF reader so they can open any PDF file you give them. I use this feature a lot especially if you want other people to see your layouts, fonts, etc exactly the way you have it on your computer.