Strong consistency

In strong consistency model all changes are atomic: they are either applied together or rejected together. When transaction is started other transactions can't modify the data because the data is blocked and other transactions have to wait until current transaction either finished or rejected.

Eventual consistency

In eventual consistency model consistency is guaranteed at some time in the future. The data may be in the intermediate state. Eventually consistent business transaction may affect many services and reach to the desired result in the end or compensate the applied changes. There is no need to block data until all operations are done.