In a sentence, diagram, or any other form of expression, the form of the expression is its written, spoken, or otherwise material form:

• Its words and their arrangement for a sentence
• The elements and their connections for a diagram
• The variables and operations for an algebraic expression

By contrast, the content is whatever that expression is trying to convey, that is, its meaning.

There are many other distinctions – signified vs. signifier, syntax vs. semantics, that come from different traditions but which for our purposes we can think of as being synonymous.