These companies invariably develop products or services, whose names they would like to protect - or, in other words, trademark. Trademarks protect names, images, devices, short slogans, expressions, and phrases that identify you, your business or your product. Corporate names and logos may also be registered as trademarks. Although generic words cannot be registered for their dictionary meanings, they may be registered for unrelated products. If used properly and renewed every ten years, a registered trademark is yours forever.
Service marks are a subset of trademarks.
Marks that do not qualify for registration in the principal register may qualify for registration in the supplemental register. Steve McIntosh of Desktop Information in Santa Barbara, CA specializes in trademark searches. Then you could check the general literature like Thomas' Register. If, for example, you are looking for a computer software product, you could also check the general computer literature by using Computer Database Plus on CompuServe and searching for your target string. TrademarkScan, Patent Research Center and British Trademarks are three online services on which you can run searches.