A blog (short for web log) is a regular column by an individual posted for all to see. There is no guarantee that anyone will read it , nor that what is written is even worth reading.
But they can benefit the marketing of an organisation both offline and online. The chief online benefit is from the links that can be created.
You can write a blog on a page on your own site or you can use one of many blogging services available. Google runs one and there seems to be many good optimisation reasons to use it.
(Because we use advanced website techniques we'll know from which page your question comes.)
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