Anyone can please explain about the difference between liquid painting and powder coating ?

As powder coating primarily is only used on metal parts, I will not discuss any actions that are needed when coating wood, plastic or other surfaces that will not respond well to powder coating.

Either way you are going to coat, it will not matter - there will need to be prep done to the parts going to be painted/coated. In the case of liquid paint, there should be no old paint, loose rust, welding slag or arc dust/smoke or oil/grease on the part - the part should be as clean as possible. Use of sand blasting, sanding, wire brushing, or even a chemical cleaner can be used - but, it must not leave any residual on the surface of the part.

Powder coating is exactly as it sounds, (it is primarily only used on metal parts) - a powder is sprayed (always sprayed, never brushed) with a charge given to it (negative I believe), at parts hung from a chain driven conveyer system that has a positive charge applied to it. The powder is drawn towards the parts due to the charge and evenly coats them. The conveyor continues and goes through a long oven that travel slowly through, allowing the powder to melt and adhere to the metal. It is just long enough that it melts the powder to a liquid, but not enough to let it run or drain off the part. It's essentially molten powder paint at that point.
When it exits the oven and starts to cool down the melted powder solidifies in a hard coating on the surface of the part - no other finishing or other coats can be applied once finished.
Generally, parts received from a powder coating facility will be wrapped in a brown paper that protects the parts while they are shipped to your location.
Benefits: - it is a very tough coating and can stand up to a lot more wear than liquid paint.
- It is also very efficient with only one coat required. (efficient in that the time spent is very little in relation to the liquid paint process - not the use of energy required to melt the powder.)
Cons:
- gives a tough finish, but surface variance can be seen in the finish as there's no way to treat it once it's coated.

It is primarily a commercial coating for heavy equipment or items being used more aggressively than home use. (example would be a metal pipe that is in a film or tv studio, where lights are clamped and moved and re-clamped all the time.

Liquid painting is spraying layer(s) of a liquid coating, (sometimes up to 5 - depending on the type of liquid paint), and allowing drying time between each layer, the drying time can be accelerated with heat lamps or a lower temp oven. Sanding, both wet and dry can be used between the coatings to get rid of any unwanted textures or other defects in the coating - this step cannot be done in powder coating due to no additional layers on top of the sanded one.)
There are generally two to three different types of liquid paint involved. (Also, there are different types of paint - latex, acrylic, oil and epoxy, described, in short, at the end).
1 - primer paint, which is used to stick to the surface of the item which, when dried, giving a good surface for the main coating paint to stick to. Primer also allows the main coating paint to dry a bit more quickly than if it was just sprayed onto bare metal.
2 - main colour layers, again, sprayed onto the part, either with a primer coating, or if the paint has a primer chemical included in it, it can just be sprayed on clean metal. This coating provides the colour and texture desired in the part and will generally be more than one coating layer... sometimes up to 3, or in specialized processes, like you'd find in automobile finishes, there can be many layers involved with different colours and or textures to give effects to the car finish. (A very expensive process.)

There can be wet sanding at this point, which will take out any runs or sags in the paint, and one last coat of colour is put on that.

Once the colour paint is done, it can be either let alone and used as is, or a top coating of clear paint is used to protect the colour paint. It will be used to protect against abrasion or UV, but can also be used to give a depth to the paint coating which is startling when you see a part done without the clearcoat and one with it)... it really adds a lot of depth in the finish. This clearcoat can be either flat, semi-gloss or a glossy finish.

There is another type of liquid paint that is more industrial and also highly regulated due to it's toxic nature - that is Epoxy paint... Epoxy paint is a two-part liquid paint process that uses two different parts that are mixed together and then sprayed onto the part being coated. It can be, and probably usually is, very toxic and is not used outside specific industries because of the requirements for safety and environmental impact should it be used improperly. Again, you wouldn't usually know about this, but it is used in special industrial coating situations.

Benefits: Liquid paint can provide a much higher quality finish than powder coating, due to the finishing steps that can be done during the process. (and also the ability to add coats should one get applied incorrectly and sanded off - very hard to do with powder coating).

Cons:
- Not as resilient to scratches or bumps like powder coating.
- Much more time involved in the process than powder coating - in general... there are some liquid coatings that aren't meant for good finishes, like primer painting only with no top coating... generally only used for structural steel to provide a more rush resistant surface.
- chemically can be quite toxic and I'm sure not environmentally friendly either in the manufacturing process.
- expensive - when using the aforementioned method of multiple layers for specialized finishes, like the colour shifting car finishes, or other mixed colour finishes - again like auto paint, "Candy Apple Red" is a good example as it uses different colours and techniques to create that colour.

I did not delve into the different types of liquid paint - such as oil (generally not supposed to be used or produced any more for home use, but can still be used in commercial or industry, or even possibly there are some small amounts of still good oil paint in previously used tins in assorted basements throughout the planet), latex, enamel and also acrylic. Generally, they will be applied in a similar fashion (brush, spray or even dipping.