Clone brands are bricks made by companies other than LEGO but designed to be compatible with some or all aspects of the system of play. As a general rule clone brands are cheaper but of poorer quality plastic and moulding than genuine LEGO bricks.
Some clone brands are blatant copies of LEGO sets and some companies have been prosecuted for intellectual property violations for their imitations.
Mega Bloks is a common North American clone brand.
Distinguishing Clone Elements from LEGO elements
Many clone elements can be distinguished from LEGO elements by their poorer quality. LEGO elements typically have a glossy finish and sharp edges. Clone elements are usually duller. The usually less durable plastic scratches more easily. LEGO elements are made to very tight tolerances while clone elements can snap together too tightly or too loosely.
Nearly all LEGO elements have the word "LEGO" imprinted on them somewhere. For elements with studs this will be on each stud. A common alternate location is the underside.