Brandon J. O'Brien explains the difference in The Strategically Small Church :
True relevance is being sensitive to the culture or subculture in which you are ministering and incarnating your ministry in your specific location. It is vital that a church be relevant by that definition...
Trendiness, by contrast, is applying a strategy that is foreign to your personality and mission for no other reason than to draw a crowd. The worst thing a small church can do is try to reach people by pretending to be like them. A church should welcome everyone; however, they shouldn't become everyone. A young person will be open to hearing the gospel from a fifty year-old pastor who speaks and dresses in a way befitting a fifty year-old man, if--and this is a big "if"--that pastor and his church show genuine concern for that young person. But this same visitor will smell phoniness from down the street if the middle-aged pastor speaks and dresses like a twenty year-old in an effort to appear "relevant."
Pastors, let's be careful what we smell like.