Soft money is the way of financing the political purposes. While the campaign finance laws limit the amount of money the political individual can apply for, wealthy entities are ready to provide them. Individuals, unions, and corporations can write a check for the solid sum of money to advocacy groups and political representatives.
As a soft money example, different political campaigns or advocating the passage of law can act. Still, the law forbids using soft money for advocating a particular candidate in an election, for instance.
The main difference between hard money vs. soft money is that the Federal Election Commission and its laws regulate hard money turnover. Soft money remains unregulated. The only law that regulates the turnover of soft money is a prohibition to use them for “party building”. Unfortunately, the law doesn’t give details about what exactly “party building” is. It’s considered though that it’s the actions that tell people to vote for certain candidacy.
When the candidacy runs the advertising and says “I am the person, who will ruin the corruption among the deputies and increase the average salary in particular states. Vote for me because…” it’s considered to be “party building” and has to be financed through hard money.