Hi Valerie,
Almost everything related to updates is tied to the timestampt of when the change was made.
Scenario #1
When the sales person changed the name to Joshua in SFDC the last modified time stamp of that record is updated in SFDC.
Marketo's sync checks for updated times stamps every 5 minutes (by default every 5 mins) and pulls the information over into the matching Marketo record. Josh will become Joshua in Marketo.
Scenario #2
When your marketer updates the name to Jonathan the timestamp on the marketo record is updated. At the next background sync this record updated will be included and pushed to SFDC. Assuming Marketo has read/write access to the First name field in SFDC then Salesforce will accept the change anJosh will become Jonathan.
We do field level conflict resolution so if between a sync field X changes in Marketo and field Y in sf.com then during the next successful sync field X is updated in sf.com and field Y in Marketo.
Scenario #3
So what if Scenario 1 and scenario 2 happened almost simultaneously, if your sales team set Josh to Joshua in SFDC and then seconds later your Marketo team changed Josh to Jonathan
If the same field is updated both in Marketo and sf.com between a sync then:
If it is a lead, Marketo value wins
If it is a contact sf.com value wins
Hope that helps,
Cathal