اجلس and اقعد are almost the same. But اجلس is more used in standard Arabic. I think "sit at the table" and " sit down at the table" gives the same meaning = اجلس على الطاولة

well there is diffrence between the 2 words : قعد mean that you are standing up and i tell you to sit then i have to use اقعد جلس mean that you are laying at your bed or somthing and i teel you to sit then i use أجلس