Brick.ie The Irish Association for Adult Fans of LEGO®

Hi everyone,
This is probably a question you have been asked before. There are so many different brands of bricks available from a multitude of manufacturers. The price variance is dramatic to say the least.

I'm wondering if they are Lego compatible. An example is Duplo bricks I assume they are compatible but what about the cheaper sets available through stores such as Tuthills, Euro Store, Dealz, Mr. Price which are all different brands mostly from China and other far eastern countries.

I'm currently planning out a model and have ordered several individual parts direct from Lego to gauge sizes etc during the planning stage, but I'd prefer to buy cheaper when possible.

(I've looked on bricklink etc and I will be using these sites)

Any information is appreciated.
Thanks
Wes