The LEC (Local Exchange Carrier) is the primary telecom provider in an area. They own the majority of the facilities and the majority of the other telecom companies provide services off of their facilities. The primary LECs are AT&T/Southwestern Bell, Verizon, and Qwest.
CLECs (Competitive Local Exchange Carrier) are secondary telecom companies that in most cases provide their service off of the LECs facilities. Some of the main CLECs are Paetec, XO, CBeyond, TW, and Integra. Also, many of the cable companies are getting involved in business solutions as well such as Comcast and Cox.
In our experience the best and most reliable services available are from the LECs, these are the companies that have been doing telecom for over 100 years and know the business inside and out. Most of the time they are more expensive, but in the long run it is worth it for your business.