Newsletter, in general, is a type of electronic publication where subscribers are notified (usually by e-mail) of new content, typically once per week. Newsletters may be Targeted at a specific audience (e.g. tech-savvy users), or may be more general in nature. Blogging is a type of electronic publication where the author posts regularly about a particular topic or issue.
Blogs can be open to all comers, or they may be geared specifically towards a certain audience (e.g. technology enthusiasts). In most cases, bloggers are paid for their content, which can lead to a more personal and engaging tone than is often found in newsletters.