A Blog page works just like a blog on your website. Once you have created the page in your space simply view it, add a new post, and write. Once it is saved the blog page will keep a running of the blog title and the first line. Your space participants can then click on the title to read the full blog.
Forum pages allow your course participants or space members to post questions and engage with each other similar to say a Facebook group. But it is housed within your space/membership site/course.
They can click write a post just like you would when creating a blog post and be prompted to add a title and body. Once published members are able to comment.
When participants comment on a forum post they have a WYSIWYG editor that allows them to format their posts and upload images as part of their comments.