(1) distinctive marks;
(2) suggestive marks;
(3) merely descriptive terms; and
(4) generic terms.
Generally, the most easily Registrable and protectable marks are those which are considered "distinctive". Distinctive marks often include arbitrary and/or fanciful terms and are unmistakably capable of identifying one applicant's goods (services) without any likelihood of causing confusion to the public. At the opposite end of the spectrum are Generic terms, which will never be capable of Registration. A Generic term is a term, the meaning of which is synonymous with the goods (services) themselves. Such generic terms are incapable of distinguishing the goods or services of one entity from those of another.
Between the two extremes lie "Suggestive Marks" and "Descriptive Terms". A mark is descriptive if it is descriptive of: the nature or quality of the goods; the intended purpose or function of the services; or the end effect upon the user. Descriptive terms can not receive Federal Registration unless, over a period of years, the terms have acquired a special, identifiable meaning in the minds of the public (that is, a "secondary meaning"). A "Suggestive" mark is one which merely suggests some quality or characteristic of the services. If the mark is determined to be suggestive, and non descriptive, the mark is entitled to Federal Registration and to the same protection as a "distinctive" mark. However, the Suggestive marks are very weak, in that they often do not provide the owner with the ability to stop others from using marks which are very similar on similar goods, or even identical on different goods.
You should make every effort to develop arbitrary and fanciful Trademarks. Suggestive terms make for very weak trademarks and should be avoided whenever possible. Descriptive and generic terms should never be used as trademarks. An effort should be made to avoid use in the trademark of terms which have a specific meaning in the relevant industry since such terms tend to make a mark either descriptive or suggestive.