A fluorescent bulb consists of a glass tube filled with neon. When an electric current is passed between the two ends of the tube (with two heating elements, one serving as an anode and the other as a cathode), photons are released which come into contact with phosphorescent material on the walls of the tube, giving off visible light.
From a halachic point of view, it might be assumed that lighting a fluorescent bulb on Shabbat is not very serious, similar to lighting an LED, but there is a big difference between the two – the way the bulb is turned on. As opposed to an LED, in order to light a fluorescent bulb the diodes at the two ends must be heated (this is the orange light visible at the ends of the bulb when it is lit). The function of the "starter" is to connect the coils at the ends of the bulb during startup and to immediately disconnect them. Thus, turning on a fluorescent bulb is the same as turning on an incandescent bulb, which is normally considered a Torah prohibition on Shabbat.
Two types of compact fluorescent bulbs have recently been developed, PL and EL. The PL type is a normal fluorescent bulb which has been made smaller for esthetic reasons, and in principle it is the same as a regular fluorescent bulb. (Such bulbs are usually connected to the electric source through two pins.) EL type bulbs, on the other hand, usually have an electronic ignition which does not require heating elements. (Such bulbs are usually connected using old-style screw sockets.) From the halachic point of view, this second type of bulb is to be preferred for essential needs on Shabbat, since according to most rabbis lighting them only violates a rabbinical decree.