Dropped
Thanks to React, in the SharePoint stack we're switching entirely to the REST API and no longer using CSOM.
Using
An xml-based query language. Heavily used in SharePoint (CSOM) for writing data queries in data retrieval web parts. Nothing complicated about it.
Examples
<Query>
<Where>
<Eq>
<FieldRef Name="Title" />
<Value Type="Text">Test</Value>
</Eq>
</Where>
</Query>
<Query>
<Where>
<And>
<Geq>
<FieldRef Name="Created" />
<Value Type="DateTime" IncludeTimeValue="FALSE">2012-01-01T00:00:00Z</Value>
</Geq>
<Leq>
<FieldRef Name="Created" />
<Value Type="DateTime" IncludeTimeValue="FALSE">2012-01-31T23:59:59Z</Value>
</Leq>
</And>
</Where>
</Query>