"Shamanism refers to a range of traditional beliefs and practices concerned with communication with the spirit world." And then lists them.
In my experience shamanic activities are not dissimilar to meditating; what can make them different is the emphasis on the natural world, drumming to induce a trance state and more structure to what happens when in that state. (For me, meditation can also be described as a trance state). The structure comes from the three worlds (Upper, Middle, Lower) that a shaman may journey through- and the fact that there is more likely to be a specific intent and focus with a shamanic journey. Meditation is often (though not always) "letting go and letting God", so the journey, while there may be an intent, is less in mind at the time.
The best explanation of shamanism that I have seen is from Sandra Ingerman. I recommend her website as an excellent place to browse and learn. And if you get a chance to go on one of her courses, take that chance. She is an excellent teacher.
To give you an idea of the sort of diversity in the notion of shamanism, take a look at Shan Rose's website- she calls herself a "shamanic dowser" and then that of Franco Santoro, an "astroshaman." An interesting site is that of Archaeology shaman, Mike Williams. For someone who emphasised the contact with Nature, Ted Andrews (now dead) is the man to research. His Facebook page is still being maintained and there are often excellent photographs of animals posted. Caitlin and John Matthews also offer training and have published a host of resources- including CDs of drumming. The American healer and shaman, Eliot Cowan, works with plants. Finally, this site- AnimalSpirits- is also worth a visit, especially if you want to check out your own power or totem animals.
Click on any of these photos for a larger version and caption.