When you take your website serious, you want to track the usage of your site. With the statistics you collect you measure how successful your website is. For example you want to measure, how often your website is visited, how often your contact form is submitted or how many orders you get from you webshop. All of these statistics you can measure with Google Analytics. When you want to also track statistics for Google Adwords & Facebook you can use the Google Tag Manager. With Google Tag Manager you can manage Tags, Triggers & Variables which give you the possibilities to measure lots of things.