You can read what has been unsealed in court right here.
https://www.businessinsider.com/michael-cohen-court-documents-plea-deal-charges-2018-8/
In a nutshell,
Cohen caused AMI, the corporation that owns the National Enquirer, to make an illegal corporate contribution by paying Karen McDougal $150,000 and then burying her
story in order to help the Trump campaign avoid bad publicity during the campaign.
It's illegal for a corporation to make a donation directly to a candidate. Donations must be made to the candidate's election committee.
Cohen used fraudulently obtained loans to open a shell company and pay Stormy Daniels to hush up.
Then Cohen fraudulently invoiced the Trump organization $35,000 a month during 2017 calling it a retainer.
The Trump organization paid the $35,000 a month and posted it to legal expenses with the notation "retainer fee".
They also "grossed up" the total amount for tax purposes.
What's not clear is, why Mr. Weisselburg was granted immunity and what kind of immunity it was.
The prosecutors in the SDNY already had most of this from subpoena documents from both Cohen and the Trump organization.
I haven't read all the pertinent documents. When they say the "Trump Organization", are they referring to his election organization or his private interest?
If the pay back to Cohen came from Trump's campaign then, yeah, that is a clear violation of campaign finance laws.
If it came from his private business, then it's a real stretch.