Taken from www.holography.co.uk
Coherent light is of the same wavelength and is in phase. A useful analogy is that of a motorway tunnel. Ordinarily a tunnel has many different types of car passing through at random intervals, this is white light. If the tunnel were 'coherent' then the cars emerging would all be the same type, and spaced bumper to bumper.
Taken from www.holography.co.uk
Interference is analogous to two stones being dropped into a pond. Ripples radiate out from each center. Where crests of ripples meet a larger crest develops, where troughs meet a larger trough forms. This is known as constructive interference. The converse is destructive interference and occurs when a crest and a trough meet and cancel each other out.