There are many 3D printing technologies, at on demand. we use a technology called Fused Filament Fabrication (FFF for short), which feeds a plastic filament, or wire, into a print head, which then melts and extrudes it onto a build platform underneath.

The plastic filament is melted as the print head moves against the build platform, extruding melted filament where needed to “draw” the object one layer at a time.

Each time a single layer has been printed, the height of the print head in relation to the build platform increases by an increment, and a new layer is "drawn", and fused to the previous layer; this until the object has been completed.

What about all the lines on the surface?

Those visible lines are called layer lines (on vertical surfaces) and tool-paths (on horizontal surfaces), and they are created by the print head moving and extruding the melted plastic needed to "draw" the object in three dimensions, they are in essence a map of where the print head has traveled to print the object.

These lines are visible to the naked eye at a 30cm distance.

This said, these lines should not be seen as defects, but as texture; and designers use this texture to augment the aesthetics of the parts they create, such is the case of the Bubble Gum Shade, designed by 3D Printed Project Athens studio, shown below, where the layer lines are used to create the appearance of a whicker weave.