What's the role of colloquiums in academia? During my bachelors and masters degree I have attended some brief talks called colloquiums in which a researcher was invited by a professor at the university and s/he gave a one-hour talk about a scientific topic most of which about his/her own research interests.

These talks are a little different from normal conference presentations and seminars because the speaker is invited to give a talk and he is not presenting a specific paper; but in a conference, except from keynote speakers; people normally submit their papers to be peer-reviewed.

I don't know what's the role of colloquiums in academia and what the speaker is seeking by giving such talks? Also, who can give such talks (a researcher, somebody from industry, an outstanding professor, etc.)?
what the speaker is seeking by giving such small talks?


One word: exposition.

As a rule, the more senior a researcher gets, the less of her/his attention is devoted to writing single research papers. It becomes more central to be known for a specific niche, a specific topic where (s)he is the world's foremost expert. One does not become such a figurehead for a specific niche by writing good papers in the area alone (although, clearly, this is still required). One also needs to be an ambassador for the niche. This includes giving keynotes at conferences, as well as giving seminars and colloquia. 


  Also, who can give such talks


Everybody who gets invited by somebody. However, typically, one does not give keynote-level talks before senior postdoc or assistant professor level, simply because most PhD students do not have all too much of a vision going beyond their thesis yet. And, frankly, the keynotes of most postdocs also suck. Like most skills in academia, giving good exposition talks is also a skill that comes with training.

(there are exceptions, of course - I can think of at least one outstanding PhD student in software engineering who was regularly invited to give keynote talks at conferences midway through her dissertation)