There is a lyric from a song by Seals and Croft that goes like this, "God made a pact with Abraham to never leave man alone". That is exactly what Baha'is believe -- that God has never left man without knowledge of Him or what is His will. Even before recorded history we believe that God has sent a succession of Divine Teachers that we refer to as Manifestations of God. Simply put, it just means that these Divine Teachers are God's Will made manifest on this, the physical plane. Their names will be quite familiar to you, some of them have been known as Abraham, Moses, Buddha, Zoroaster, Krishna, Jesus Christ, Mohammed and most recently Baha'u'llah, just to name a few. 

Their teachings can be divided into two parts, the spiritual truths they reveal are essentially the same throughout history and can be best exemplified by what we know as the "Golden Rule" in the Christian faith, a version of which exists in every religion. These spiritual truths extend to all manner of spiritual teachings and only change by virtue of the subtlety with which they are expressed. 

The second are social teachings, which change according to the times. The social teachings are progressive since man's sophistication and awareness has grown and will continue to grow through the ages and they are perfectly suited to the times and to man's needs. Examples of these might be ordinances regarding marriage, inheritance or dietary laws.