Pre-Islamic Turks believed in Tengrism. The common misconception is that Pre-Islamic Turks were Shamanist. Even though large numbers of Turks practiced Shamanism, the main Religion of the Turks was Tengrism.
"Tengri" in old Turkish literally means "God". I think that The Tengri Religion was pretty simillar to Islam in a lot of aspects (Judgement Day, Tolerance towards other Religions etc.)
However it was also different in a lot of aspects too. For example Ülgen is the son of Tengri he's the khan of Heaven. And there's Tengris other son: Erlik Han khan of the Underworld. There are also different Gods and Godesses.(Islam completely rejects the idea of Polytheism or God having children).
The Gods of Tengrism are very diverse. There are good ones, bad ones, chaotic ones etc. IMO they're pretty interesting.
Tengrism today has largely been replaced by Islam but not completely extinct. It is still practiced in Turkic countries such as Kazakhstan.