Blog (also known as CLOG or the Collaborative Log) allows members of Sakai worksites to post entries to a worksite log. The default view of the tool shows the posts of all the worksite members, chronologically ordered, and this reflects the primary supporting use case of the tool as a collaborative logging tool. In a worksite, Blog can also be used as a more conventional blogging tool in that you can select a particular author from a list and view only their posts. Blog also supports use from a user’s My Workspace where he/she can work on a private, personal log or make their posts public, whereupon they will be visible in to anybody in the world from a Gateway placement of the tool.
Search here for how-tos on the Announcements tool.