English from Old German "all".
1. As a given name it was introduced into Britain by the Normans. The variants Ala and Ela are from that era.
2. Ella was also a short form of a number of Old German names beginning with Ali- which means "foreign". Examples of surviving derivatives of those names are Eleanor and Elvira [q.v.].
3. Pet name for Ellen and hence a derived given name.
4. Ella is also a popular diminutive for a number of girl's names ending -ella, a few of shich are listed above. For a full list search on "*ella".