The difference between classic regular and slim fit is mainly in the cut and shape of the t-shirt. Classic regular-fit t-shirts have a looser and more relaxed fit. And on the other hand slim-fit t-shirts are more form-fitting and tailored to the body. Slim-fit t-shirts are suitable to accentuate the body shape, while classic regular-fit t-shirts offer more comfort and ease of movement. You can also check this blog Athletic Fit Vs Slim Fit, to get an idea of which One is Right for You.