CAML Query with Person/People/Group columns

<Eq><FieldRef Name='Supervisor'/><Value Type='User'>Bob</Value></Eq>
- to retrieve list items where Supervisor field has value "Bob".
- "Bob" is the username

<Contains><FieldRef Name='Leads'/><Value Type='User'>Jane</Value></Contains>
- to retrieve list items where "Jane" belongs to the multi-valued people column Leads
- "Jane" is the username
Important! The condition above will actually return list items with the following values in the column Leads: "Jane", "Jane Doe", "Test Jane".

To avoid this situation, use the following instead:
<Contains><FieldRef Name='Leads' LookupId='TRUE'/><Value Type='User'>18</Value></Contains>