Is a "spy" different from an "informer"? If the answer is yes, what differences are there?
Yes, they have different meanings. A spy is more likely to be a sanctioned agent. It's their job, or at the very least their function.
An informer is less likely to be employed in their position. Sometimes it will be a member of the criminal organization who feeds information to others (law enforcement in particular), but it could also be a neighbor or a girlfriend or some other kind of witness. Some cops pay their informers (also "informants") but this isn't necessary to the definition. "Informer" is more loosely defined as one who informs. "Informant" is more specific to informing authorities.
While an informer may spy on activities, that doesn't make them a spy per se.
"The informer spied the unloading of the illegal cargo. He informed the spy of these activities so action could be taken."
A spy is (usually) collecting information for an organization or a person; he could also be doing it for himself. However, a spy can also work to cause harm to something without gathering any information (e.g., many WW II spies never gathered information but provided bad information to the enemy to trip them up).
An informer is someone who provides information because he has either worked for the organization (e.g., Edward Snowden) or someone on the outside who has knowledge gained by observation and/or by a reading of the facts (e.g., someone informing on a medical professional for Medicare fraud based on personal knowledge).
When we think of the word 'spy', we think of someone used in time of war to gain INFORMATION on the enemy while imparting this information to their own side for use in conducting military actions. When we think of the word 'informant', we think of someone in the civilian sector (usually a criminal) who has agreed to gain and give information to law enforcement officials to facilitate a desired outcome against a criminal action or entity. BOTH terms involve INFORMATION. BOTH terms involve transferring this information to parties outside of the entity from which the information was acquired. A SPY is an INFORMANT. An INFORMANT is a SPY. In the end, it's all a matter of semantics.
I dealt with drug informants for about 15 years and taught classes on confidential reliable informants. They are basically the same. Spies usually provide information. Informants can provide info or buy drugs, stolen property etc. People equivocate spies with Jason Bourne, James Bond etc. Those who try to find a difference are simply trying to sugar coat the difference if there really is one.