A linear regulator is a device to take a varying input voltage as input, and give a constant voltage output, even though the current demanded by the load may vary. There are important caveats: the output voltage is LESS than the input, always, and current will have some practical maximum. These devices still obey Ohm's Law! If you like, you can think of them as "resistors that change their value automatically to keep output voltage constant". They do this by using transistors, which together with diodes, are the most fundamental types of "active component" (semiconductors). You will need to have a decent understanding of these to get much further. Linear regulators are not that efficient - they just burn off excessive power as heat (because they operate basically as "smart resistors"). These devices usually have three terminals - in, out, ground.
A buck regulator is another type of voltage regulator which uses a "switching" mechanism to do the same as a linear regulator, but in a much more efficient way. They do not have to burn power to adjust output voltage (at least, very little) - they use a "feedback" mechanism to keep it constant. (When you study amplifier circuits you will come across feedback a lot.)
There are also boost regulators, which can give out more voltage than they get in! How? using special properties of diodes, transistors, capacitors and inductors. When you understand the more basic elements well, you will be well placed to study this. Until then, a rough idea of what (buck and boost) converters do is enough.