Emotivism says that a moral claim such as "abortion is murder" or "you have an obligation to repay your debts" is not a sentence with a truth value. It is, instead, an expressive utterance - such as saying, "Abortion, yuck!" or "Three cheers for repaying one's debts!"
Another component of emotivism is that the emotive outburst is meant to cause others to share the same reaction. In doing so, it does not use reasoned argument from true premises. Instead, it uses some form of coaxing or emotional persuasion to get others to have the prescribed attitude towards that which is being evaluated.
Desirism begins by questioning the claim that emotive statements lack a truth value. For every emotive statement, there is a corresponding truth-bearing proposition that says the same thing.
For example, Jose is offered a lunch consisting, in part, of refried beans. He turns up his nose at the refried beans and says, "Yuck." The claim is that emotive utterance has no truth value. Yet, it corresponds to the proposition, "Jose hates refried beans." This latter statement does have a truth value.
Furthermore, it is just as objectively true or false as a statement of the form, "Jose has a scar on his wrist from when he fell off of his bike."
Desirism denies that moral statements are merely expressions of personal likes or dislikes - arguing that they are instead expressions that concern malleable desires that people generally have reason to promote or inhibit. However, even if they were merely expressions of the likes and dislikes of individuals, they would have a truth value.
The issue of whether moral claims express personal preferences will be handled separately under the subject of "subjectivism".
On the question of whether moral claims are utterances that attempt to cause in others to acquire a particular emotional reaction to a state of affairs, and to bring about this attitude through something other than reasoning from true premises, desirism holds that this is true.
Moral utterance are not only truth-bearing propositions, they are - at the same time - an attempt to use the social tools of praise and condemnation to effect a change in the emotions (desires) of others as they relate to the object of evaluation.
The purpose of moral praise is to act on the reward mechanism in the brains of others to strengthen the desires that lead to the praised act and a desire for the praised act itself. The purpose of moral condemnation is to act on the same system to promote aversions that would motivate agents to avoid the type of act being condemned.
Thus, we have the use of tools other than reasoning from true premises (praise and condemnation acting on the reward system) in order to cause others to adopt a particular attitudes (desires). This is a primary function of moral praise and condemnation.
At this point it is important to note that praise and condemnation not only serve to alter the attitudes of the person being praised or condemned. They have an effect on others. Mirror neurons cause people to experience events as if they are happening to themselves rather than others. They are a key component of empathy. If a person cuts his finger, another who merely witnesses the cut will have a mental reaction that is similar to having his own finger cut.
Consequently, praise not only reinforces the desires of the person praised. It also reinforces similar desires in those who witness the praise. This explains why moral praise is often delivered in front of an audience - through award ceremonies and public testimonials - so that the praise can have an effect on society as a whole.
The story of praise or condemnation does not even have to be real. One can make up a story in with a character who is praised or condemned for some action and use that to mold the character, at least to some extent, of those who engage with the story. Thus, parables become a useful tool for teaching moral lessons.
However, none of this calls into the question the possibility that the moral utterance - the act of praise or condemnation - can, at the same time, be a truth-bearing proposition.
The factual component of moral claims, combined with their ability to impact desires even as stories, makes it possible for people to sensibly use moral claims even in situations that traditional emotivism found hard to defend. For example, making moral claims about past events (e.g., Churchill had no right to promise eastern Poland to the Soviet Union) still reports a fact (a person with good desires would not have done such a thing), and can still have an impact molding current desires to make it less likely that a current or future leader will perform a similar act.
Consider the case of an emotional utterance, "That is a lie!". This is a truth-bearing proposition. It states that the accused agent asserted to be true something that the agent at the time actively believed to be false. This statement itself is either true or false. It is possible to refute the claim, "That is a lie," by showing that the agent sincerely believed what he said.
Yet, the proposition, "That is a lie!" can, at the same time can be delivered with a certain emotive force. One can be clear, from tone and context, that the person who says, "That is a lie," is, at the same time, condemning the person who is uttering the lie, and urging others to have the same attitude towards that type of behavior.
This condemnation does not change the truth-bearing nature of the proposition being uttered. The statement is still either objectively true or objectively false. Furthermore, it can well be the case that, if the accusation was false, the condemnation would not be justified. It is not the case that an utterance must be cognitivist (truth-bearing) or non-cognivist (incapable of being true or false).
It can be both.
Moral claims are both.