Plasmids are often used to purify a specific sequence, since they can easily be purified away from the rest of the genome. For their use as vectors, and for molecular cloning, plasmids often need to be isolated.
There are several methods to isolate plasmid DNA from bacteria, ranging from the miniprep to the maxiprep or bulkprep. The former can be used to quickly find out whether the plasmid is correct in any of several bacterial clones. The yield is a small amount of impure plasmid DNA, which is sufficient for analysis by restriction digest and for some cloning techniques.
In the latter, much larger volumes of bacterial suspension are grown from which a maxi-prep can be performed. In essence, this is a scaled-up miniprep followed by additional purification. This results in relatively large amounts (several hundred micrograms) of very pure plasmid DNA.
Many commercial kits have been created to perform plasmid extraction at various scales, purity, and levels of automation.