Astral Projection is also known as an OBE (Out of Body Experience) this is where the spirit/consciousness temporarily leaves the physical body. Some people would argue that Astral Projection and OBES are different. Astral Projection being able to travel to other worlds, OBES being limited to the earth plane, but from my experiences there is no difference. The astral plane is not a place as we know it, but a higher vibration of existence, so if you're out of your body, then you're out of your body and what you decide to do while you're out is up to you.
So in reality Astral Projection and OBEs are the same, an OBE is the name given to a spontaneous experience, it just seems to happen so to speak, where as Astral Projection is a forced experience, you make it happen by using various techniques. People often confuse dreams of flying with an OBE, you will know when you have had a real OBE because you will often feel a buzzing sensation and when your spirit/consciousness returns to the physical body you will feel a jolt like an electric shock, the intensity of this jolt will vary depending on your own biological makeup. Although dreams of flying can be used to trigger an OBE especially if it's a lucid dream.
OBES (a spontaneous experience) are generally unpleasant experiences this is due to the nature of the vibration differences in the physical and spiritual realms. When we leave our bodies we are entering into a higher vibration and we are not used to this vibration, although this will bring no harm to you please use a protection exercise before you start, it can be quite frightening, it was for me when first tried it. Fear can actually stop you from experiencing an OBE, I still get scared when it happens although not so much as I used to. Astral Projection (a forced experience) is not so unpleasant, although the feelings are the same they are a lot more subtle, but can still vary in intensity depending on the individual. The reason these are not so unpleasant is because they are gradual and the vibration adjustment is gradual unlike an OBE where you're suddenly out of your body and haven't had the time to adjust to the vibrations, thus making them more intense and unpleasant.