(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
This consideration usually weights non-profit or education use over commercial use. This means that commercial uses are less likely to be seen as a fair use of the work.
This is where the idea of a tranfromative work comes into play as well. A use that is somehow changing the work or uses it in a new way are more likely to be fair use. Mere reproduction of a work is less likely to be considered a fair use but still can be depending on if it passes the other considerations.
(2) the nature of the copyrighted work;
Whether the work is creative or fact based is one of the biggest things looked for when deciding this factor. Fiction or creative works are usually seen as having more protection under fair use, while fact based or non-fiction based work give more leeway. Also work made educational purposes is more likely to pass this consideration. However, courts are very protective of unpublished, such as private correspondence, and tend to rule against fair use.
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
The less of a copyright protected work you use the better and the more likely it is to fall under fair use. There is not hard and fast rule for how much you are able to use, there are many out there saying that you are only able to use 10% or so many pages but this is not part of copyright law. Sometimes use of the entire work will still fall under fair use. The amount must be looked at within the context of the work because what you are taking is considered outside of the amount. An example would be that taking a small quote from a book would usually be acceptable but if you take the climax or big revel you may still lose this factor because you took the most important part. The best way to deal with this factor is to only take the amount you need and really think about whether you are including more than you need.
(4) the effect of the use upon the potential market for or value of the copyrighted work.
This factor looks at how your use of the work will effect the commercial or other gains of the author. Will your use of the mean that people will use your work over the original or does your use mean the author misses out of a potential market. This one is very complicated but does not favor use in a commercial capacity.