T1 can refer to many types of connections. In the telephony realm, a T1 is a dedicated phone connection supporting up to 24 simultaneous calls. From a data perspective, a T1 can provide rates of up to 1.544M bits per second. Each channel of the T1 service can support one individual call, or 64K bits per second of data or internet traffic.
While often replaced by newer coax and fiber options, T1 is still used frequently and is still a standard for highly reliable service. T1 usually comes with some service level agreement enabling higher priority in case of disruptions.
Since providers lease these last mile connections, costs can vary by location and provider.