Theology is literally "the study of God"; it is "Faith seeking understanding" (Anselm)
The rich history of theology gives us a full and fairly consistent language in which to express our understanding of God. To that end, let us get a few major terms out of the way first. After that feel free to browse the links below.
This is something which is known only through revelation -- not through human reason or experience. It is not the 'we don't really have an answer' fallback, but the knowledge that we do not fullyunderstand. We speak of the mystery of God similarly to the way we speak of the mysteries of Space, not like we would an Agatha Christie novel. Something to be learned not something to be solved. It is from the Greek mysterion and is translated into the Latin as sacramentum. It speaks of the connection between the sacred and the profane as well as their separateness.
Faith is with a capital 'F' and is a noun, not a verb. Faith is the sum of all revealed truths. Because we have Faith, we are able to believe. Having Faith means that you have access to the Truth. Faith is not, as some posit, the belief in the unprovable. 'Provable' is, like 'Verifiable' a word which means different things to different people. When we reduce the universe to human knowledge alone and exclude Revelation, then 'verifiable' is only that which I can sense or reason out; thus is the universe limited.
Belief is the action of Faith. It is not blind, nor is it irrational. Belief is a form of active knowledge, that is, when we trust, we believe some truth. Not to be confused with Faith but analogous to having Faith. When we reduce Faith to belief, then our only Truth is what I can believe; thus is God limited.
This is the rational and emotional knowledge of a thing. It is not complete knowledge, but perhaps the 'common-sense', sufficient knowledge of the thing. This is the end goal of theology.