Illegal string PDO fetch() vs fetchAll()

When I try to call fetch() after executing a PDO statement, I get the following warning:

Warning: Illegal string offset

It works perfectly when I use fetchAll(), though.

What's the difference between the two, and how would I use either one?

$allData = $con->prepare("SELECT * FROM users");
$allData->execute();
$result = $allData->fetch();