A statement is a single piece of work, whereas a batch is a whole set of work.

So, for example:

DECLARE @Id INT;

SET @ID = 52;

SELECT *

FROM dbo.MyTable

WHERE Id = @Id;

That is one batch with three statements. You'll see a difference in execution results between the batch as a whole and the SET statement. Usually, unless you've got very specific troubleshooting going on, you don't want to collect statement level information using Profiler. It's very processor intensive.