What is Balast

It would take a few pages to explain this completely, but here is the
short answer.

I assume you know that the main purpose of a ballast is to regulate lamp
current, and two secondary purposes are to provide the starting voltage
(which is higher than the running voltage) and in some lamp types, to
provide power to the lamp electrodes.

A magnetic ballast is composed of passive components such as inductors,
and perhaps capacitors and perhaps transformers, all of which operate to
the power line ( or mains) frequency. This is 50Hz or 60Hz, depending
upon where you live. 

An electronic ballast contains an electronic inverter that generates
power at a frequency that is usually above 20kHz. The power line (or
mains) power comes into the ballast, it is usually converted into DC
power and then the inverter takes the DC power and converts it to power
at a frequency above 20kHz. The main reason for using an electronic
ballast is that fluorescent lamps operate more efficiently (about 10%)
when operated at frequencies above 20kHz than they do at power line
(mains) frequencies. Secondary reasons for using electronic ballasts are
that they are smaller and lighter than electromagnetic ballasts and they
can be easily dimmed.

An electronic ballast contains an electronic inverter that generates
power at a frequency that is usually above 20kHz. The power line (or
mains) power comes into the ballast, it is usually converted into DC
power and then the inverter takes the DC power and converts it to power
at a frequency above 20kHz. The main reason for using an electronic
ballast is that fluorescent lamps operate more efficiently (about 10%)
when operated at frequencies above 20kHz than they do at power line
(mains) frequencies. Secondary reasons for using electronic ballasts are
that they are smaller and lighter than electromagnetic ballasts and they
can be easily dimmed.