What Adventists believe you could summarize under two headings: (a) what they have in common with many other Christians; and (b) what is specific for their own faith community.
Adventists believe in the triune God: Father, Son and Holy Spirit. Jesus Christ, the Son of God, is for them the Redeemer who is at the same time fully God as well as fully Human. The Holy Spirit is the third godly Person who supports us continuesly in our faith live.
God spoke to mankind via the Bible, the inspired book which forms the basis for all our fundamental beliefs and the guide for our way of living.
But God has revealed himself even more directly by becoming a human being. Jesus Christ came to earth in a supernatural way. He lived there, and suffered and died for humanity. By his act of love an escape route was made available and eternal live came within our reach. The Christian lives by hope that Jesus, as he has promised, wil return to this world to make an end to all suffering, decease and death. Many developments are ‘signs’ that this event is quickly approaching.
Adventists also emphasize, as other protestants, the value of the ordinances of baptism and holy supper.
The most striking difference with other Christians is the fact that Adventists have their day of rest on the seventh day of the week (Saturday, Sabbath) and not on the first day (Sunday).
From the start of their history Adventists have emphasized the importance of Biblical prophecies. In their interpretation they differ on many points with other Christians.
Slowly the differing point on the state of the dead is accepted by other Christians. Adventists do not believe in the immortal soul, who lives on after death independent from the body. But they believe that someone who dies enters a state of ‘sleep’, untill at the second coming the dead are raised from the death.
‘Stewardship’ is an important subject in the Adventist Church. God is the owner of all things, also of our financial means. He takes first place and Adventists return ten percent of their income to the Lord. Stewardship also includes to take care of the body which we received. Healthy living, refraining from damaging substances (e.g. tabacco and alcohol) and certain foods are some of the ways Adventists express this value.
Seventh-day Adventists do not believe that they are the only Christians that will be saved, but they are convinced that they have certain insights which most other Christians lack and that they therefore have a special task to proclaim that to everyone in this world. They fully commit to that.
For a more information on what Adventists believe: 28 Fundamental Beliefs.