a site with a single connetion to ISP is called single homed
either use static routes, or advertise the site routes to the ISP and receive default route from ISP
when customer is connected to SP via static routes, traffic from that customer on Internet is indistinguishable from traffic from the ISP – you are not ISP independent
in this environment it is not recommended to run BGP
Dual homed site
customer connect again to one SP but via two links on one router or two links on two routes or single link on two routers to the ISP.
it brings more redundancy
same as single homed site, i.e. you can use static routes or advertise routes to ISP
in this environment it is not recommended to run BGP
Multi-homed site
customer is connected to two ISP.
this gives you ISP independed solution – you run BGP, you got AS number, you can route your public networks via whatever ISP you choose, you can manipulate the routes however you want
in this scenario bgp is typically used. You can manipulate the path, do some path manipulation what networks are reachable via what ISP.
You can exchange with ISP whole routing table of just some networks (ISP own networks or networks to some known ASses you want to reach)
Dual multi-homed site
same as multi-homed just with another redundant link to ISPs