There are many factors that the USPTO uses to determine if there is a "likelihood of confusion" between an existing mark and a mark sought to be registered (see TMEP 1207 et seq.), but, in general, a likelihood of confusion exists when one uses the same or similar mark that another is already using and uses it on goods or services related to those sold by the senior user.
This standard means that the two trademarks do not need to be exactly the same or used on the same goods or services to trigger a likelihood of confusion refusal by the USPTO.
For example, the similarity between how two trademarks are pronounced, even if they are spelled completely different, can still cause a likelihood of confusion refusal. Unfortunately for the applicant that filed WEEDIES for "[b]reakfast cereals" on August 7th, this means his application is doomed and his filing fee lost.
Since these two marks are phonetically equivalent, and the applicant for WEEDIES filed in the exact same class and for the exact same goods contained in General Mills' Wheaties registration (which has been using the term on breakfast cereals since 1924, by the way), this application will be receiving an office action refusing to register WEEDIES when it is assigned to an examining attorney at the USPTO in approximately three months. The applicant's $225 filing fee is almost certainly lost.
How do you reduce the risk of a likelihood of confusion refusal from the USPTO? Work with a trademark attorney who can conduct a proper search and identify any possible conflicting marks before you file your application. Otherwise, your nonrefundable filing fee is in jeopardy.