A lecture is a format of education presented as an oral presentation that teaches people about a specific topic or topics that the person delivering the presentation is normally somewhat of an expert in. Usually the format chosen by schools, colleges and universities though it is not too dissimilar from the way a politician, minister or even a businessman will conduct speeches. Lectures are not considered to be the most effective form of teaching and often receive criticism, but as of yet a practical alternative is yet to be found.
Lecturing is considered a very passive form of learning as it only involves the people being taught to listen and answer the occasional question and is very one-directional. Lectures delivered by practised and talented public speakers however can be very stimulating and engaging, while maybe not active they will get you thinking. The format of a lecture delivering information possibly to hundreds of people does not lend itself to active learning however is effective at introducing large numbers of people to a topic.
As mentioned, the effectiveness of lectures is a highly debated topic however there are some advantages to the format which include; quick exposure of new material to a lot of students, potential for engagement (student willing), grants the lecturer control of the class and facilitates communication between class members. Lecturing is also a great place to reveal very up to date or unpublished information not available anywhere else.