Transparencies used along with slides also enable you to have more simultaneous information before the audience - there are times when there's a legitimate need to do this. Yes, provided you aren't too obvious about using them. You should familiarize yourself with the material in advance to the extent that notes are unnecessary. However, even though you don't need the notes, it's psychologically comforting to have them available just in case you forget something. Even very good speakers occasionally have to refer to their notes - it's not looked down upon provided it isn't done too often.
Seminars are generally longer talks given to a broader spectrum of scientists.
However, the basic principles of preparation and presentation apply equally well to both seminars and research presentations.