There is more to active listening than simply not talking. Active listening means paying attention to what lecturers and interlocutors are saying, no matter how hard it is to understand them. Often, the most effective listeners actually interject frequently, because shrewd listeners will try their best to get clarification on points. The difference between effective and ineffective listening is much larger than many people imagine.
The listening process begins before you start physically listening to someone else speak. A good listener will use pre-listening strategies whenever possible. For example, if a good listener knows he has a class on 20th century philosophy on Wednesday, he will read up on the subject before attending the lecture. He may rely on encyclopedias and textbooks to get the most basic and authoritative take on the subject. Once he has done his reading, a good listener may try to ascertain what the speaker's purpose for speaking is, giving him an them advantage in contextualizing what he hears. Inneffective listeners, by contrast, won't do any reading beforehand.
In the "during" phase of the listening process, effective listeners give complete attention to the speaker and take notes on what they hear. They take notice when the speaker says that a point is especially important and paraphrase that point in a notebook or on a computer. Ineffective listeners, by contrast, may daydream, doodle or text message while they are supposed to be listening. If an ineffective listener is trying his best, he may simply attempt to write down everything the speaker says, an effort that isn't likely to succeed.
Responding is a crucial part of the listening process. In one-on-one conversations, an effective listener will participate actively, interjecting remarks and asking for clarification. In lectures, an effective listener will ask questions whenever the speaker leaves the floor open for questions and will always raise his hand and wait to be called on before asking. An ineffective listener will simply tune out his interlocutor in a one-on-one conversation and recklessly interject irrelevant comments in lectures without asking for permission to speak.
The listening process doesn't end when active listening ends. After listening, effective listeners will review all the notes they took and write a one to two paragraph summary of the presentation based on these notes. Ineffective listeners, by contrast, will either not do anything after the presentation/lecture is over, or try to memorize all their notes verbatim. Whether due to no effort or overly laborious effort, the ineffective listener doesn't end up with any long term understanding of what he heard.