Exactly which of the “indicia of incorporation” are appropriate to a given business is a function of state law. Most (all?) states require that a company use such an indicator so that people dealing with the company understand that they deal with an incorporated entity and not an individual. Most states allow a typical corporation to get by with Co. or Inc.; Inc. also is appropriate (at least in some states) for non-profits. If the business you dealt with used “Ltd.,” your home state may allow (or allowed at the time) such a designation; I’m not aware of any state which does, but I only know about a handful. (I did look at Missouri and Kansas, and Ltd. doesn’t appear to be allowed in either.) Also, the company could be a foreign corporation which had registered to do business in your state; many states allow a business to use its regular designation even if native companies aren’t allowed to use that same designation. Otherwise, the business was in violation of the law. (Some U.S. states appear to have allowed Ltd. to apply to a limited partnership, although traditionally that is referred to in the U.S. as “L.P.”) This isn’t exactly uncommon with small businesses who don’t know that they have to register with the state corporations agency and don’t understand all the rules they must follow – he might have just been making it up to sound snazzy.
The reason corporate form is important is, as discussed above, because it limits the liability of the investors. If you’re a sole proprietor (that is, you personally own a business), or if you’re a member of a traditional partnership (you and a pal personally own the business), and your business fails, the creditors can come after your personal assets. Because legally, you are the one who entered into debt on behalf of the business (such as to buy inventory, lease space, employ staff, etc.), the same way you might have entered into debt to buy a house or go to college.
A corporation, OTOH, is legally a “person” separate from its owners. So if you incorporate, you put money into the business, and if it fails, you’ve lost what you put in, but you have no personal liability to the business’s creditors, because they contracted with the corporation, not with you personally. This is why it is critical that a potential customer, vendor, landlord, etc., must know whether it’s dealing with a sole proprietorship or a corporation – so they know whether their deal is backed by the assets of just the business or with the assets of the business + all your personal assets.
Traditionally, corporations had one major drawback – so-called “double taxation.” (This is a misnomer.) When you’re a sole proprietor, you own the inventory, you sell it, you pocket the cash and you’re done. But with a traditional corporation, there is an additional person. Essentially, you fund the corp., the corp. buys inventory, the corp. sells inventory, then the corp. distributes the profits to you. Since you and the corp. are two different people, you both have to pay taxes – the corp. on the money it makes from doing its business, and then you on the money you receive from the corporation. This additional taxation is the price you pay for liability protection.
However, in the last few decades, efforts have been made to lower the tax burden on small business in an effort to encourage them. The easiest way to do this is to allow them the liability protection that comes with corporate form without having to suffer the “double” taxation. An L.L.C. is currently the most common form, accepted in many states. This is a small corporation which doesn’t suffer from double taxation. There are both state and federal requirements that an LLC must meet, typically including a size limit. “L.L.C.,” which does stand for Limited Liability Corporation is also a misnomer – all corporations offer limited liability; that’s the very point of corporate form. An LLC should actually be called a limited taxation corporation, because that is the benefit it has over traditional corps., but I guess that doesn’t have the same ring to it. 
That’s why I posted above that David Simmons’ definition of a “Ltd.” corporation is the same as our regular Co. or Inc. – it offers limited liability by interposing the existence of an additional “person” between those who fund the business and the debts it enters.
Of course, this whole thing is in a bit of cocked hat know because of the current Administration’s tax changes. The new tax law purports to remove double taxation by making the distributions from businesses non-taxable not just for LLC’s but even for big companies. If such a change becomes permanent there would seem to be no need for LLC’s or any other type of “pass-through” entity. It seems unlikely that these tax changes will persist (I’ve got my fingers crossed) but who knows?
–Cliffy