To tack on some more non-lawyer info onto Stormynature's post:
1) Does an unregistered trademark give you complete authority over the name of your trademark. No it does not
2) If someone has a trademark on "Company Name," am I allowed to put a trademark on "CompanyName?" Yes and no. If your business is distinctly different (as in you make bricks and they create games) then probably yes however trademark laws usually have enough flexibility within them that an argument might be successfully made that you are infringing regardless.
3) How does an unregistered trademark expire? Normally Trademarks expire when no longer being used commercially but this is becoming less and less the case these days - it might almost be safer to assume that trademarks don't expire rather than open a can of worms (but best to see a lawyer on that).
I've seen a ton of website with an unregistered trade with "Coming soon" on the page, and they sit there for 10 years. Oftentimes establishing who was first with a trademark can be point on which a trademark's ownership is allocated, there are usually laws within the trademark acts where you can also "reserve" a trademark which may not be utilised straight away but does afford some of the protections under trademark law.
The key thing to understand with trademarks is that trademarks and patents and copyrights serve different purposes.
The goal of patents/copyrights is to encourage inventions/content-creation, by letting inventors/creators know that they can profit from their inventions/creations without their work getting stolen by other companies and produced for cheaper. It encourages sharing of discoveries, sharing of knowledge, and sharing of culture, by giving inventors and creators a free period of time to profit before their work becomes open-hunting to every company. By then ending the copyright later, the government allows competition in a freemarket to drive down prices, further benefitting consumers.
(Note: I'm not happy with what I feel is overly long lengths of copyrights, but copyright length is not important to the conversation).
Patents and copyrights are intended to indirectly benefit the public by directly benefiting companies.
Trademarks on the other hand, are meant to directly benefit the public, and they just happen to also benefit companies.
The purpose of trademarks are to prevent customers from thinking they are buying one product, when they are really buying another inferior/cheaper product. It's to prevent the customer from getting ripped off - so there's zero reason to put a timelimit on them.
Companies benefitting from them by allowing them to invest alot of money in marketing and building a brand is an effect but not the goal of trademarks.
Mario as a Nintendo trademark will never expire, but Mario as a Nintendo copyright will eventually expire.
This means, Mario will eventually (in a bajillion years*) be available for others to create their own Mario games and call them "My Original Mario Name" (after it expires), but that calling your game "Mario Galaxy 5", misleading consumers (intentionally or not) into thinking your product is an official part of the Mario Galaxy series will still be trademark infringement, for the protection of the consumers buying products.
Trademarks = For consumers directly, so they last forever until they are no longer in use.
Copyrights/Patents = For creators directly, for consumers indirectly (by motivating creation), but will expire so it benefits consumers directly (by competition lowering prices).
Part of the original purpose of copyrights expiring is to keep monopolies on important parts of culture from existing and to allow culture to spread and encourage the translation of works into new languages, and permit new artists to expand upon older works of art.
*Mario expires in 2077, in the United States, if copyright isn't extended yet again - but copyright has been continually extended in the past 50 years or so, so nothing prior to 1923 has fallen out of copyright. Other countries have longer or shorter lengths, but usually recognize the longest international length anyway.
What's to stop someone from creating a million unregistered trademarks so that others can't have them? Probably nothing at all, except in practical terms the lack of use of these trademarks would most likely be successfully challenged with very little difficulty as none of these trademarks are likely to be associated with any commerce beyond the trademark itself and have no "reputation" associated with it.
In addition, unlike patents, the more 'everyday use' your name is, the harder it is to defend. So unlike with domain names, everyday used words aren't good choices. Combinations of words are good, and new words or creative mispellings of existing words are easier to enforce. The numerical amount known as a Googol was what the company Google was originally trying to name itself, but they accidentally misspelled it on their trademark registration form (they were college students, and forgot to look up the correct spelling before they went to register it) - their accidental misspelling actually increased the strength of their trademark.
Also, if you trademark a normal word, such as 'Delta', and decide to create an airline, your trademark only applies to your industry. Others can also use the word (because it's not an original word) for a different industry... such as hotels. Though the Delta Airlines and Delta Hotels is slightly more unusual, because they are both in related industries (both in broader travel industry), nothing is stopping me from, say, creating a software company or product using that common word.
If you're abusing your trademarks just to sue others, and your trademarks are overly broad, they can be revoked. As, thankfully, Tim Landel found out recently found out. (He was trolling companies for lawsuit settlements).
Also note that trademarks aren't just words: You trademark your company name, logo, and even noises that are used to promote identify your brand (MGM lion roar, Mac startup noise, XBox startup noise, etc...). Noises and images that aren't used to identify your brand are copyrighted, not trademarked.
Hopefully that extra background info gives you a better perspective on the purposes and uses of trademarks.
