What does it mean to enter the US legally?

i have some Ghanaian pals who entered the US on an F-1,took Delta Airlines,ped tru JFK and USCIS and now in America,are they illegals if they want to find work and study at the same time?do u people refer to those Mexicans,Hispanics etc who cross the border without pports nor visas as those whom u always refer to as illegitimate illegals?Are they not the people who annoy Americans the most?Are f-1s holders likely to get a job in the US hussle free as compared to b-2s?Is it difficult for an F-1 to get a job(even just as a restaurant server etc)?