The total number of trunks a business has determines how many people within the business can simultaneously be on the phone with callers outside the organization. Standard trunks can be both inbound and outbound. The telco sends the set of destination digits for incoming calls down the DID trunks to allow the businesses phone system to route calls. DID trunks allow inbound calls only. Lines are the actual wires that connect the local telco to the business.
Extensions are the physical phones within the business.
T1 is a special set of lines that allows 24 logical trunks to be multiplexed on the same line using analog signals. On a standard residential ISDN line, there are typically two BRIs, thus allowing two simultaneous phone calls. On a PRI ISDN circuit, there are 24 BRIs, the same capacity as a T1. Demarc is the area of demarcation where the telco-owned wires stop and the internal building wires begin. All office buildings have a phone or utility room somewhere, maybe just a small closet, with a demarc in it.