The textbook definition of a political action committee is an organization formed to collect money and provide financial suppoer for political candidates. PACs may be used to buy access to a candidate and influence in legislature. There is no direct correlation between PAC contribution and votes on the floor but that doesn't mean the two are not connected at all. PACs are also used by candidates, before announcing candidacy, to raise money in order to support their campaign travels and other related expenses. They are used as a means to an end for developing contacts, raising money, and votes for a cause.
I, personally, think that PACs can be positive, but as with all things in politics, they have become very corroded. Big business and the generally wealthy can give the most and do, to see to it that their "needs" are met. Also, the way politicians use PACs to support their campaign unofficially then "return the favor" once in office - not very ethical or truly helpful in the long run. All the while, there are smaller, and likely, poorer organizations, many of whom who may have a more "honest" cause, who cannot contribute nearly as much and whose influence will be drowned out by big business and wealthy personal interest. And Congressional candidates have no public funding to campaign, so (a) PAC(s) may aid slightly less wealthy candidates. So you see, PACs actually can be a very positive thing.
On paper, as with many things, again, in politics, PACs sound like a good idea and a nice, non-profit way to pay for the costs of campaigning or help out groups who are serious about their cause being heard out but our government is run by people - particularly, politicians. Politicians, generally, have agendas and people are selfish and have ugly ways; so, what was a good, clean idea on paper becomes a raging machine that sucks up money and resources and spits out things like "access" and "influence" but if you don't feed it enough of this... "money" - you won't tend to get too much from it.
[A short essay I had to write for my government class. Sorry. Fixed the typos!]
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It seems that a lot of political institutions look great on paper and are pretty screwed up in real life. Funny how that works...