When Amazon RDS creates a snapshot, it is consistent with the moment in time when the snapshot is started. Any changes after that time will not be reflected in the snapshot.
Amazon RDS uses Amazon EBS for disk storage. Thus, the snapshots are presumably EBS Snapshots. (I say 'presumably' because this level of detail is not published, aside from saying that the snapshots are stored in S3 the same as a normal EBS Snapshot.)
All backups are both full and incremental. It's best to ignore those terms with EBS Snapshots because it's just an implementation detail. (If you wish, you can read-up on the topic. It has to do with incrementally backing-up modified disk blocks, but full backups are always available rather than needing to apply 'full data' and then 'increments'. It's the cloud -- it's magical!)
Bottom line: You don't need the traditional fears associated with backups since AWS does all the "undifferentiated heavy-lifting". It just works. (But you're welcome to test it!)