Mockup generators are usually online services that make customizing your t-shirt and adding your t-shirt design as simple as selecting a shirt color and uploading your image. The trade off over using a template for a software program like Adobe Photoshop is that t-shirt mockup generators are usually less customizable. This can make it difficult to get your image exactly like your final printed t-shirt and in general give your less options on your final design (tags, texture, background).