The connector that runs from your WiFi card to your antenna is known as the "pigtail." Because different manufacturers produce different connector sets for their WiFi cards, we need to know what type of connector we need to buy.
When you are buying your wireless kit, you notice the page has a dropdown to select the pigtail type. There are a number of pigtail types listed on the page, and if you find your manufacturer, pick the corresponding type. If you cannot find it or you want to verify it, check out this pictorial list of connectors. Look where you would plug in an external antenna on your WiFi card and match it up with the picture that looks like it would plug/screw in to that connector. The one that matches is the one you need.