No seams are completely invisible, but there are many factors that can contribute to just how visible they are.
Certain types of carpet like Plush and Shag styles will have less visible seams. Seams in Berber, Level Loop, and Patterned carpets are much more evident.
The light in the room makes a difference, too. Carpet should be laid in the same direction of a natural light source whenever possible (like a large window or door) because the light can have a tendency cast a shadow over the seam.
No seams are invisible, but the proper layout can effectively minimize their appearance.