How is it different from the algebraic expression in Example 1 (Refer to NCERT page no. 16)?          [Page No. 17]

Example 1 was $4x + 5y + 3$. There are two key differences:

Example 1 has a constant term (3), but Example 2 has no constant term.

In Example 1 every term uses a single variable ($4x, 5y$). In Example 2 the term $50lw$ multiplies two variables together.

Because of that $lw$ term, Example 2 is not a linear expression, that single term has the two variables multiplied, giving it a higher combined degree. Both expressions, however, use two variables.