What is asterisk \ how does it work?

i’m new here and recently have been searching about asterisk , after a small search about voip and pbxes.

i have recently met this solution to an ip centrex.
first of all just to confirm could asterisk supply and be an answer to the needs of a 2 line ip centrex will all pbx functions (accept from the voip line which i understand should be supplied by our local carrier?)

i ahve several wuestions about asterisk and the way i works

first of all i understand the asterisk is a sort of a software that should be installed on a server which will be a part of the network while all phones should be connectedto the server is that currect?

i also know that asterisk should supply all functions of a pbx system is thath correct?

moreover, i would like to ask how do you manage all the functions? is i through a web service?

what are the diffrences between the free asterisk to the business?

how do i config each phone that is connected to the server.
could a pc in the network be the server in the system or should there be a new server for this kind of a system ?

thank you , i hope i didn’t bother any of you and really need some answer lol