re: What type of tree is this? Looking to line entire backyard fence

Could be Italian cypress but they are generally more narrow and taller than those in your pic. There are several varieties of juniper such as "spartan" and "hetzi" that are columnar so it could be one of those. Italian cypress are finicky and have several pests and diseases that attack them so can be tough to keep them looking good for more than a few years. Take a look at Taylor juniper which is a columnar variety of eastern red cedar and more hardy than italian cypress.