Listening may not seem like a mystical or complicated process: someone speaks and you listen. As you receive the information being communicated, you're likely not realizing that you "decode" the dialogue, interpreting it according to your known information and set of beliefs so it makes sense to you. Misunderstandings occur when that interpretation differs from what was intended by the person speaking. Listening barriers are one factor in communication misunderstandings.
According to McGraw-Hill's "Communicating at Work," physiological barriers that hamper our ability to listen are hearing deficiencies and rapid thought processing. A person who has difficulty hearing will either miss critical information or will be so focused on trying to hear the words that his energy is diverted from processing it. Rapid thought processing involves trying to process the information too quickly.
If you've ever tried to listen to a speaker in an auditorium when the air conditioning system failed on a hot summer day, you know how challenging it was to pay attention when your body was uncomfortable. This is known as an "environmental barrier," and it can significantly hamper your ability to listen to someone, since you're distracted by your own discomfort. Other barriers include noise and information overload.
Your own attitudes that you bring with you have an effect on your ability to listen. Even the fear of seeming ignorant by not asking for clarification on something you don't understand is an attitudinal barrier -- you can't properly decode information that doesn't make sense to you. Other examples are impatiently waiting for your turn to speak, thus paying more attention to "getting your turn" than to listening properly and being preoccupied with your own thoughts.
If you travel to a foreign country and native speakers converse with you in English, you should be able to understand them, right? Odds are, you'll find it difficult. Part of the reason is that their English is spoken with a heavy cultural accent that still sounds foreign to you. The longer you're in the country, the more attuned you become to this accent and can decode the information more easily. Other cultural differences include the use of pauses and even the rate or flow of conversation, which can both be listening barriers.
Sometimes our gender roles can cause problems with listening; for example, by competing for attention in a social or business situation. This can happen with social flirting or in business if a woman feels like she has to "fight harder" to achieve a certain position. Listening can also be improved through training and education, and not having those opportunities can create a barrier in your communications.