"A giant worm, that created the Ancient Ways. It ate through the rock, leaving lapis lazuli behind.”
whereas Angie Sage said that
"The Orm is actually a giant wurm and it only ever lays one egg."
We know that Orms are reptiles because they are relatives of Dragons and Septimus first described them as "snakes" which built the Ancient Ways. Due to their close similarities Orms are most likely related to Land Wurms but this has yet to be confirmed in the books but Angie Sage did call Orms - giant wurms - on her blog.
It is also known that the Akhu Amulet, a Magykal source of power for all ExtraOrdinary Wizards, was made from lapis that came came from the belly of the Orm. Lapis Lazuli from a great Orm is also called Orm Lapis.
Lapis Lazuli concentrates Magyk like nothing else and can make even minor Magyk powerful according to Marcia Overstrand. Which is why an Orm under a Wizard's possession is so dangerous because the Lapis it creates can be used to increase a Wizard's power.
Orms can be imprinted to a Human just like their Dragon relatives which can make them potentially very dangerous. Dragons feel kinship towards Orms and will protect their eggs as shown in PathFinder when Spit Fyre follows the Orm Egg in order to protect it. Orm Eggs can also come with Enchantments for example the Egg of the Great Orm gave the Enchantment that covers the Eastern SnowPlains with beautiful snow throughout the year and allowed the people of that land to live in towers of ice until the Egg was stolen by Oraton-Marr. Though he did say that the Egg was frozen by the Enchantment of some misguided people to imprison it and prevent it from hatching.
The Egg of the Great Orm was laid in a Hub made entirely of Lapis Lazuli in the Eastern SnowPlains and the Egg itself laid in a the Orm Tube inside the Hub (which is apparently the last burrow made by the Magykal Great Orm). The Egg of the Great Orm (Princess Driffa described it as a Lapis Egg) is described as being large and egg-shaped as well as being brilliant blue streaked with gold and glows with light unlike Dragon eggs which could take on many colours and tended to be small in comparison.