Depending on context, these two expressions could have similar meanings. The difference lies in by whom "it is understood". "We understand" is the equivalent of "it is understood by us". If the "by" phrase is omitted, then the use of passive voice indicates general understanding. The speaker could be saying "everyone understands" or "everyone knows". It is a rather arrogant statement, in my opinion, and a good example of why we should avoid passive voice.

The difference is that the latter emphasizes the subject, "we", the former doesn't, as any other pure passive patterns. But obviously, the subject is people. Sometimes "is it understood?" is less polite then plain asking "do you understand?", sounds like a lieutenant asking his soldier.

the former is more objective and common-use i believe