Legos is a term often used in North America, particularly by those outside the LEGO hobby, to refer to LEGO brand building elements or LEGO sets.
As a registered trade mark, the term 'LEGO' is an adjective, not a noun, so the usage is technically incorrect (just as "kleenexes" is incorrect, it should be "Kleenex tissues"). The LEGO Group, as outlined in their Fair Play Policy, discourage use of the generic term legos as it has the potential to dilute the brand name. For example people may use legos to apply to Mega Bloks brand bricks.
The use of the term appears to be restricted to North America. There are no reports of the term in common usage outside of the United States and Canada.
Some fans think the distinction is petty, while others carefully try to honor it, so it can be a bit of a debate starter or a running joke. Making too fine a point of it can alienate people, but to some purists, the term grates the ears.