Here is a really useful blog how historians can make use of Twitter. It summarises well those 5 tools and gives examples.
The 5 tools are:
1. Follow organisations: get updates & research tips, highlights or additions to collections, get in touch with staff. These could be museums, libraries / archives, research instutions, history organisations, research group, etc.
2. Use hastags: to search tweets by subject, identify an interest group
3. Tweet a conference: share thoughts on talks, etc. with conference participants, great for networking
4. Share resources: new resources, recommendations, etc. are regularly posted.
5. Search for job
@HFLOxford is where we tweet updates about library services and collections for historians, new resources/books/databases, circulate news about tools for historians and generally pass on any news that makes lives for historians easy, richer and keeps it up-to-date.
The full descriptions are at AHA Today: Five Ways for Historians to Use Twitter.