XLT and Lariat are two different trim lines, different standard features, different options available. Most of the difference is found in the cab: trim materials, seating features, electronic accessories. In the F-150 the model lines are XL, XLT, Lariat, King Ranch and Platinum. In the past they've had "sporty" and "off-road" trims like FX4, those may be options.
There might be as much as $20,000 difference in the price of a mechanically equivalent Platinum vs the XL or XLT. I've always bought my trucks at the XLT trim level, or another closely priced to XLT but without carpeting (e.g. in a Ranger these have included "Sport" and "Edge."
SuperDuty pickups may have different trim levels. I know that cab-chassis models used to build working trucks usually don't offer the top two or three trims. Towards the end of the model run, for example, the E-series came only in XL or XLT, with a variety of interior trim upgrade packages.
Whether shopping Ford, Ram, Chevrolet, GMC, Toyota, Nissan, you should become familiar with the respective trim lines. Not knowing, a salesman can easily get an extra $5000-$10,000 out of you by selling you up to features you don't actually need, and would not want if you understood how much extra you were paying to get them. When I was shopping for my van, the salesmen kept finding me examples that included a $4000 radio/navigation package; eventually saved $18,000 by buying a low-mileage ex-fleet example with just the options I needed.
Ford has been pretty stable with names, the XL - XLT - Something Special combination has been used for at least 30 years. Dodge/RAM lately has been changing trim names and sometimes changing the meaning of old names. At Chevrolet, they are either LS or LT but there are two or three levels of LT. You might also find, limited to a few cab size/bed length combinations, a W/T (for work truck) that competes with Ford's XL in fleet sales. Chevrolet does more with option packages than with model lines.
Toyota and Nissan, you will find feature availability more closely linked to trim level. At Ford, Chevrolet and RAM, something standard on an upper-middle trim might still be available in an option package on the lower trim truck. This is less common on "import" brands, where a limited option least helps simplify manufacture and inventory management for product manufactured and shipped overseas. This practice seems to carry into the "import" models manufactured in the U.S. as well, which includes the full-size trucks, mid-size and larger cars, and minivans; just the way the corporation has chosen to do it.
When shopping used, 1-3 years, I find that SuperDuty Ford trucks (and equivalent Dodge/RAM and Chevy/GMC) are often much less expensive than many "1/2 ton" models of same age and similar mileage. This is because the majority of these heavier trucks are sold initially into fleet, or at least commercial, use and the first buyers don't buy the expensive luxury trim and accessories. The lighter trucks include a lot more trucks coming off lease for personal use, at the more expensive trim levels.