Conspiracy theories and propaganda tend to go hand in hand, mainly because many conspiracy theories tend to be against something (government, industry, etc) while at the same time trying to promote something (political beliefs, alternative medicine, etc). Sometimes it's difficult to tell which is which. Sometimes they are one in the same.
Some conspiracy theories are obviously propaganda and are generally not believed by most (including those that promote the conspiracy theory) except for those who are to deluded (or have deluded themselves because of some personal bias against a certain person or group) to not accept the conspiracy theory/propaganda, or are not smart enough to figure out the deference between what is real and what is a lies.
Examples of this would be holocaust denial and the allegations the President Obama wasn't born in the United States.
Then there is the flip side of this in that some conspiracy theories have been used as some sort of propaganda. In fact it's not that uncommon that certain politicians, world leaders, and even certain celebrities will use these already established conspiracy theories in order to obtain power, maintain power, and just get attention for themselves.
Sometimes the leaders of one country who is experiencing hostilities from the leaders of another country will use already established conspiracy theories in there propaganda about their rival country as a way to try to make the leaders of that rival country to back off, and to perhaps try to sway the people in the rival country over to their side.
Probably one of the best examples of this would be the 9/11 conspiracy theories, in particular with Iran's president Mahmoud Ahmadinejad, who has most likely used the 9/11 conspiracy theories as a way to gain support and downplay the United States, and as a way to get the US to back off from Iran.
Then there are of course conspiracy theories that you cannot tell if they were propaganda that became conspiracy theories, or if they just started as conspiracy theories, and ended up becoming propaganda. In fact many of the anti-government, modern medicine, new world order type of conspiracy theories tend to fall into this category.
This is actually where many conspiracy theories fall under simply because it's difficult to tell whether or not a conspiracy theory began as propaganda that evolved into a conspiracy theory, or vice verse. Examples of this would be FEMA camps, chemtrails, and vaccines causing autism (along with a lot of other things).
Then there are of course conspiracy theories that are so bizarre that most skeptics (and even some conspiracy theorists) suspect that they may have originally began not as propaganda but as a either a hoax or a scam.
Regardless of whether or not a conspiracy theory originally started as propaganda, or if a conspiracy theory is being exploited by someone with an agenda, the fact is that conspiracy theories are used as propaganda, and we should be skeptical of certain people or groups and ask ourselves this: are they promoting this because they seriously believe this, or do they have some type of agenda.