The main difference betwenn a painting or study and a print is that the paintings and studies are one-of-a-kind items. Prints on the other hand are usually one of a number.
 
This can be a single print, usually called a 'Unique' by the printmaker and will be signed and numbered as 1/1. this is in fact a 'painting' produced by way of either lino-cut, silk screen or other form of hand printing, and really should be seen in the same light as an original painting.

 Other prints are 'limited edition' prints, and have a set number produced, these should be signed and numbered as above, but with the first figure the number of the print and the second number the total of the edition. These are less valuable than single or unique prints which would have the same value as an original painting. The degree of the reduction in value usually depends on the size of the edition.

Finally you have the plain 'print', these are not really worth anything other than the cost of production etc. and will not hold or increase value.

Paintings are self explanitory, and provided they have some artistic content and are not constantly churned out by the artist will have a fair chance of being an investment.  Oil paintings tend to be more collectable as investments, this is traditional, although acrylic is seen as a viable medium nowadays and are collected as investments also. Traditionally watercolour paintings don't fetch the same prices, this is for two reasons, the first is that watercolour is less permanent, due to the paper/base fading and discolouring (watercolours are transparent, not opaque) and secondly; traditionally, watercolours were seen as sketches or studies and not as 'finished' works, thus were of less value.

Studies, as the name suggests are paintings or sketches usually done on site, or 'en plein air' and are used by artists as information and basis for other more 'finished' (finished off works would have more work involved, and would be of better detail and quality) paintings. These studies are usually sold for a lesser amount by the artists when they have finished with them, and are an ideal way to start collecting Artists who tend to fetch a higher price for thier work. Artists who would be recognised as collectable, usually sell these studies through thier studios rather than through Galleries, thus studies are often listed as 'studio sale' items by SRA's (self representing artists)

 Studio sale items sometimes include older finished works too, and are worth while looking out for as they are often sold at a reduced value to clear space. Ebay and other auction venues are a good place for artists to sell these, as only current work tends to go into Galleries.