If you do a search, there are many threads like this one, and I have personally posted one myself.
The definition tends to vary from company, sector etc but IMO is as below:
1st Line Support - Helpdesk (Job titles such as Helpdesk Analyst, First Line Support, Call Logger)
Typically involves resolving calls over the phone in ans ervice desk environment and may escalte to 2nd line support.
2nd Line Support - Desktop Support (Job titles such as IT Technician, PC Engineer, Desktop Support Analyst)
Typically involves onsite desktop support of all workstations, printers, scanners etc
3rd Line Support - Network Support (Job titles such as Netowrk Admin, Network Manager, Server Engineer)
Typically involves onsite support of all servers and dedicated to maintenance of network related issues.