rentrez provides functions to search for, discover and download data from the NCBI's databeses using their EUtils function.
Users are expected to know a little bit about the EUtils API, which is well documented: http://www.ncbi.nlm.nih.gov/books/NBK25500/
The NCBI will ban IPs that don't use EUtils withing their
user
guidelines. In particular /enumerated /item Don't sen
more than three request per second (rentrez enforces this
limit) /item If you plan on sending a sequence of more
than ~100 requests, do so outside of peak times for the
US /item For large requests use the web history method
(see examples for entrez_search or use
entrez_post to upload IDs)