The RapidKL LRT (Light Rapid Transit) system was formed by the merger of the former Putra and Star lines. The old names are still in common use, so it's helpful to know that the old Putra line is now called the "Kelana Jaya" line, and the old Star line is now called the "Ampang and Sri Petaling" line.
The lines are still poorly integrated with each other; at Masjid Jamek, the only transfer point, you have to exit one station, walk down the road, enter the other station, and buy a new ticket.
Tickets are sold from vending machines and human-attended booths in the stations. Despite the fact that most fares are between RM1 and RM2, the machines do not accept RM1 notes, so you must either have or want a pocketfull of coins in order to use them. For this reason, the attended booths are popular and the queues are often long.
In order to avoid the queues, you can get a Touch'n Go card, which you can load with any amount of value and then use on the RapidKL LRT, KL Monorail, and KTM Komuter systems, as well as motorway tolls and in many car parks.