Generally, when you say, "I agree with you," you mean that you heard and understood what someone else said and that you support their conclusions. Agreement could be understood to mean the same as the equals (=) symbol in mathematics.
When groups are voting as a process for making decisions, generally speaking, most understand the question to be about agreement; i.e., "Do you agree with proposal A or do you oppose proposal A?" In the voting process, this question is asked of each person participating, they cast their answer as a "vote" and the votes are counted and the answer that receives the majority of the votes wins the decision.
The word Consensus is based upon the concept of Consent rather than Agreement. The focus of Consensus process is in a different arena than the process is in a discovering which idea can gather the most votes. Consensus process focuses on discovering what decision would be in the best interests of the group to which every participant can give their consent.
In the Consensus process, you are never asked if you agree or not; the question is: "Do you have any concerns that must be resolved before freely giving consent to adopt a proposal?"
This is a substantial concept that affects every aspect of Formal Consensus. This is why you will not find the concept of "building Consensus" [on this site]. The decision-making process known as Consensus is an alive, dynamic, and constantly evolving process before the Decision is made. This is the part that could be called "building Consensus"; however, generally, the phrase is used to describe something that happens after a decision has been made and the decision-makers want to convince others to accept it.
In a formal setting, Consensus Decision-making process starts with the introduction of a proposal or issue from an individual or group. Then there is a time when individuals can honestly and safely express their ideas. Then, the group has an opportunity to creatively address the conflicts and everyone Cooperates in finding the resolution that best serves the common purpose of the group. It ends when there are no more concerns that need to be resolved before the decision is adopted; i.e., the group has "reached Consensus" or given their consent to adopt the proposal as a decision. Once a decision has reached Consensus, the next phase is to implement it; there is no further need to "build Consensus" if it was truly a Consensus process to begin with.
Sources: Consensus for Cities, On Conflict & Consensus
Contributors: Amy Rothstein, C.T. Butler
Recommended Books: Consensus for Cities, On Conflict & Consensus