Trademark infringement centers on consumer confusion.
If a consumer is led to believe by the name that one product or service is really that of another product or service, due to the intentional naming or an inadvertant naming on the part of a "second" company, it can lead to an infringement suit filed against this second company by the trademark-owning first company. This confusion can arise through company names that are strikingly similar, product or service names that are strikingly similar, or domain names that are strikingly similar.
Infringement suits can be expensive actions to defend against so it is always wise, before investing a lot of time, money and energy into a business, to check to make sure that your business is not infringing on the intellectual property rights of any other company.
It is usually best that you come up with a name for your product or service that is unlike any others. Invented words work well (think Kodak and Xerox and Kleenex) as do portmanteau words (think Microsoft). The unique name can tell consumers that your company is unique, as well. And another plus is that a unique name protects you from an infringement suit.