What happens when records are merged in Salesforce?

I'm wondering if anyone can tell me what happens on the Salesforce side when a records are merged. Are the old records technically "deleted" in Salesforce so that only the merged record remains? Should merging in Salesforce also merge the records in Marketo?

We have over 170K records in Marketo that were deleted from Salesforce, and I'm wondering if these could be leftover from merges that occurred in Salesforce.