Questions about SHARES

Hello Miners ! Silly questions, but as a beginner I am still lost. I have configured my PC to perform some tests on a mining pool, and finally have been able to connect.

GUIMiner says 4.2Mhash/s , but the mine pool says : 7.158Khash/s … that’s really different… which one should I trust ?

GUIMiner said : 1 share accepted at 07:00:00PM … Sorry, but I have no idea of what is a share, and if I should get much more than 1, or not.

Thanks for your help !
Have fun mining !