Variables in Google Tag Manager allow you to pull in data from your visitors’ online experiences. A tag like a Google Analytics pageview doesn’t need to be provided with any more additional data; however, a tag designed to fire when a conversion occurs will tend to need more data like the order number or conversion value. The dataLayer provides these values to you, you will use variables to access this data in Google Tag Manager.
Google Tag Manager has a number of different variable types; refer to the Google Tag Manager documentation for an explanation of those types. Normally, you will be using either a dataLayer variable variable or a custom JavaScript variable to access data on the page. The dataLayer exists to describe the sales flow in a consistent and supported way. By supported we mean that if we make changes over time we will keep the dataLayer up-to-date with those changes. In some cases, you may well find that the data you want to work with in a tag is not available in the dataLayer. In many cases, it will be possible to access that data using custom JavaScript. Just keep in mind when you do this that at some point the page or the flow may change and your variable and tag will no longer work.
