Ancestry contains records for the United Kingdom, Ireland and the United States. It does have resources for other countries too but there are far fewer. The records span from the 14th century almost up to the present day.
It is a useful stepping stone for researching your family history and discovering useful dates, names and locations, however, you will have to use your initiative and possibly spend some money to find other important information like certificates or records.
You must be in a Westminster library to access Ancestry, either on a library computer on on your own device via the libraries' wifi.