What is "confusingly similar"?
Confusingly similar, or "likelihood of confusion", is a test in trademark law used during the examination process to determine whether a mark conflicts with an earlier registered mark, and is also used in trademark infringement cases to determine if using the mark infringes upon a registered trademark.
Where a mark is not identical to a registered mark, the use of a mark may still constitute trademark infringement if it is "confusingly similar" to the registered trademark.
One awesome example that Wikipedia mentions, Microsoft has become such a public name and trademark, other businesses in the industry would highly unlikely be able to use the term "micro" or "soft" in their names. Since Microsoft generally does not hold exclusive rights to the words independently, it would need to prove that any business names or trademarks which include these terms would be confusingly similar to "Microsoft". "Microsafe" or "Micro Software", although clearly different would more than likely be considered confusingly similar and be enough to constitute trademark infringement.
In addition, the style, logo, or font, can become relevant. For example Google utilizes specific colors with the letters and Microsoft products are distinguished in the marketplace by a their font among other things. American Express uses a proprietary font.
Infringement cases of this type can be proven with surveys that show members of the public who are likely to use the services or goods have been confused by it. However, Courts can also rule that an infringing mark meets the "likelihood of confusion", if it is obvious to even a casual observer.
Parody and satire does not meet the confusion standard. The point of a parody or satire is to bring the registered mark to mind and poke fun at it. A parody or satire would not work effectively without being able to bring the original mark to mind. For more information on this, see the Fair & Nominative Use section.
One such parody is the logo created by the One Step Behind website who we obtained permission to reprint their British Petroleum satire trademark logo and really love the level of "transformation" they were able to obtain with the rendering. There is no way this logo meets the confusingly similar standard. This is 100% fair use.