You can adjust a site's conversion window length in your Attribution Settings. These can be found under Settings->Attribution Settings. A "Conversion Lookback Window" indicates how long your campaigns will count conversions for after a view or click occurs.
With a 7-day post-click Conversion Lookback Window, for example, if someone clicks an ad and converts 7 days later, that will be counted as a conversion. If someone clicks and converts 8 days later, it will not.
Post-click conversions are conversions that occur after someone views your ad, clicks on it, and converts on your site.
Post-view conversions are conversions that occur after someone views your ad, does not click on it, but later returns to your site on their own and converts.