A blog is an authoritative voice in the community. A blogger (or team of bloggers) distributes information in a blog post to an audience who's involved with the post subject matter. Blogs are best suited to providing announcements, news, experience accounts or expert opinions on the subject of the blog. They and their data are tied to the group where the blog is located. (The following capture shows a news-related blog post.)
A blog is about distributing focused information. The only exchange with readers is in the comment section, and the comments are not considered as important as the blog post. A blog is most appropriate when you want to talk about experience, news, or information from a point of view. In a forum, both Q&A and discussion posts can be allowed, and the respondents are as important as this the askers. In a wiki, you are recording information in articles for people to reference. In a gallery, you're providing files to view or download.
Please follow the instructions in the What is an application? article.