In Google Adwords, conversion tracking can be done in following ways:
Tracking purchases by involving basic tracking code provided by Adwords and modifying it with additional code unique to your particular e-commerce platform
View-through conversion window options tracks when a person sees your ad but does not click it (impression)
By enabling your ad rotation setting to optimize for clicks, meaning that Adwords will serve the ads it feels are likely to be clicked
By accessing search funnels inside tools and analysis, you can also know when customers clicked on your ad for the first time and how frequently they saw your ad before converting.