The Ensembl project was started in 1999, some years before the draft human genome was completed. Even at that early stage it was clear that manual annotation of 3 billion base pairs of sequence would not be able to offer researchers timely access to the latest data. The goal of Ensembl was therefore to automatically annotate the genome, integrate this annotation with other available biological data and make all this publicly available via the web.
Single search engine of NCBI. Entrez is not a gene database. It’s the name of the NCBI infrastructure which provides access to all of the NCBI databases. One of those is the Gene database, so you would say “Entrez Gene”.
there is not necessarily an one-to-one mapping between Entrez Gene and Ensembl Gene IDs.