Hard to say. I would not really call it pixel art, but painted sprites (pixel art often use dithering, reduced color palett etc). It is likely that they use a painting tool like photoshop for painting sprites.

It looks like the characters at least are pixel art, but definitely not pallet-limited and probably heavily edited in Photoshop or the like.

is Ragnarok's characters and monsters Pixel Art ? and which software do you think they used for it ?

Yes, those are hand drawn. Either on paper, or directly into the art program with a wacom tablet. The specific software doesn't matter, it's all traditional art skills.

Pixel Joint would be the place to look up things about pixel art. From their forum: Free online Pixel Art games

Pixel art is not a clearly defined term. IMHO pixel art is the creation process and not the final product. Like oil painting, you can create, or better say fake, the look of a oil painting with digital tools, but an oil painting is only an oil painting if it is painting with the right technique (oil paints).

Pixel art is from an era where resources were limited, especially resolution and colors. When painting images for small resolutions (16x16 or 32x32) you start adjusting single pixels to get the image right. You just can't use photoshop and a brush to paint them, you will need to modify pixel for pixel, which is where the name of pixel art is coming from. Therefore many pixel artists consider MS paint more useful than photoshop.

Ragnarok online have quite large sprites, therefore it is unlikely that they places pixel for pixel. Most likely they paint it with some kind of paint tool like photoshop and apply some filters (outline) to give it a certain charm, but at most it is faked pixel art.