Judging by the names of these servers you are asking about the QlikView famility of servers:
Enterprise Edition Server: the regular QlikView server for any size company for internal company deployment. Includes the option of having multiple servers in cluster, to run QlikView Publishing Service separately from the application server(s).
Small Business Server: meant for a limited number of licenses internal for company deployment. Lacking scalability features and license extensions above a certain limit.
Test Server: for companies that want to separate Production servers from Test servers. This server type is meant for a more sophisticated application deployment with Development, Test, QA and Production in separate environements.
Extranet Server: A server type to allow users from partners and or customers of a company to use applications deployed on it.
Information Access Server: A server type that allow full internet access to the server for any kind of user. Will function as a webserver that runs publicly available QlikView applictions.
QlikView Expressor Server: an ETL-server of the now discontinued QlikView Expressor familiy. Not sold since 2015 I believe.
Most of the servers are more licensing options although some functionality differs, the application development and ability to host any QlikView application is almost 100% identical.
