1.) A trademark is any word, phrase, or design, or less commonly an alternate "device" such as a sound or scent, used to describe a commercially sold product for the purpose of distinctively identifying it to relevant consumers. Sometimes you may hear people refer to “servicemarks.” A servicemark is the same thing as a trademark except that it is used to describe a commercially available service instead of a tangible product.
2.) Trademark rights are established by use - and not by registration. Registration has the potential of enhancing rights that already do, or will, exist via the following advantages: your trademarks can become easier to enforce in the event of litigation; it becomes easier to expand the geographic scope of your trademark protection; competitors may be less likely to infringe on your trademark and may also be less likely to accuse you of infringement; and you may be able to establish priority dates, both in the U.S. and abroad, before a product is ready to be sold commercially.
Just because a trademark is not registered does not mean that no trademark protection exists. Just because a trademark is registered does not necessarily mean that trademark protection does exist. The USPTO does not even conduct a full trademark search or analysis when deciding whether or not to register a mark - the USPTO only searches its own database of registered marks.
3.) On occasion, you may also hear the phrase “descriptive mark.” A descriptive mark describes the type of product or service it refers to, and not merely the company or brand offering the product or service. Because, by definition, a mark must "distinctively identify" a good or service - descriptive marks are generally considered weaker than non-descriptive marks, and can be tougher to register as a result. But that hasn’t stopped some people from achieving very meaningful trademark protection for their descriptive marks (i.e. Holiday Inn, Jiffy Lube, etc.)
4.) "Generic marks" are like descriptive marks but taken to an extreme. You cannot call and apple an Apple and expect "Apple" to distinctively identify the product you are selling. You can call a computer an "Apple" and that is not considered generic (unless consumers have come to equate all computers with the word apple). "Generic marks" are not subject to any protection.
5.) Trademark infringement is determined on a "likelihood of confusion" basis. The issue at hand is whether relevant consumers would be likely to associate the goods or services of one party with those of another party as a result of trademark usage. A variety of factors may be examined when analyzing a trademark infringement issue, the most notable of which are: the similarity - visual and phonetical - of any marks at issue; and the commercial relationship between the goods or services described by any marks at issue. There is no clearcut manner for determining trademark infringement.
6.) When registering a mark with the USPTO - which can be done yourself but a professional should be able to avoid common mistakes - the cost of registration will depend on how many International Classifications you file in. Often, particularly among inventors, it is only worthwhile to file in one class, although there are probably some exceptions.
7.) When registering a name with the USPTO, you can file a standard character mark or a stylized mark. The benefits of a standard character mark are: you can use your mark in commerce as a trademark in any combination of styles/fonts/cases/etc without losing the benefits of registration. The benefits of a stylized mark, where a certain style is well decided upon: the mark becomes more distinctive and therefore the USPTO may be more likely to grant registration where there are otherwise questions; your scope of protection can become slightly broader as there are more elements making up your mark, and therefore there are more ways in which other marks could be considered confusingly similar to yours.
Often, the ideal way to go is to file in both a standard character form and separately in a stylized form.