A Lutheran is “a member of the protestant church that observes the Lutheran Confessions and some of the writings of Martin Luther (1483-1546),” although that doesn’t immediately seem to answer the question of what Lutherans believe.
Instead, let’s start here: a Lutheran is a Christian who approaches their Christian faith in a Lutheran way.
First, as Christians, we believe in a Triune God: a God who created all things as good; God’s Son, Jesus Christ, who granted life through his death; and God’s Holy Spirit which moves actively in our lives and throughout the world.
Yet, as Lutheran Christians, we emphasize three aspects of this faith:

As a congregation of the Evangelical Lutheran Church in America, Zion Lutheran Church is a welcoming, inclusive church that shares a living, daring confidence in God’s grace and love. Come and experience Zion—and see how we live out this Lutheran Christian faith.
http://www.elca.org/What-We-Believe.aspx
“Lutheranism 101″ http://www.thelutheran.org/article/article.cfm?article_id=5895&r=417%20target=