Valyrian steel blades are incredibly strong, incredibly sharp and keep the edge very well. It's possible that the metal is also created using spells. The steel is folded lots of times to remove the impurities too, similar too Damascus Steel (note that a lot of properties of Valyrian steel are similar to Damascus steel).
The properties of Valyrian steel are well-known, and are the result of both folding iron many times to balance and remove impurities, and the use of spells—or at least arts we do not know—to give unnatural strength to the resulting steel. Those arts are now lost, though the smiths of Qohor claim to still know magics for reworking Valyrian steel without losing its strength or unsurpassed ability to hold an edge. The Valyrian steel blades that remain in the world might number in the thousands, but in the Seven Kingdoms there are only 227 such weapons according to Archmaester Thurgood's Inventories, some of which have since been lost or have disappeared from the annals of history.
The World of Ice and Fire, Ancient History: Valyria’s Children
Later on Maester Yandel compares Dawn to Valyrian Steel again commenting on it's strength and sharpness.
The Daynes of Starfall are one of the most ancient houses in the Seven Kingdoms, though their fame largely rests on their ancestral sword, called Dawn, and the men who wielded it. Its origins are lost to legend, but it seems likely that the Daynes have carried it for thousands of years. Those who have had the honor of examining it say it looks like no Valyrian steel they know, being pale as milkglass but in all other respects it seems to share the properties of Valyrian blades, being incredibly strong and sharp.
The World of Ice and Fire, Dorne: The Andals Arrive
Bran remarks that Ice is spell-forged, dark and sharp.
Lord Eddard Stark dismounted and his ward Theon Greyjoy brought forth the sword. "Ice," that sword was called. It was as wide across as a man's hand, and taller even than Robb. The blade was Valyrian steel, spell-forged and dark as smoke. Nothing held an edge like Valyrian steel.
A Game of Thrones, Bran I
Jon remarks that Valyrian steel is light and sharp.
Jon dismounted. Slung across his back in a black leather shoulder sheath was Longclaw, the hand-and-a-half bastard blade the Old Bear had given him for saving his life. A bastard sword for a bastard, the men joked. The hilt had been fashioned new for him, adorned with a wolf's-head pommel in pale stone, but the blade itself was Valyrian steel, old and light and deadly sharp.
A Clash of Kings, Jon II
The blades are dark grey and contain a swirling pattern from where it has been folded back on itself. The spells in the metal somehow remember the colour it should be and attempts to change it usually prove fruitless, though Widow's Wail and Oathkeeper retain some red in them.
Tyrion wondered where the metal for this one had come from. A few master armorers could rework old Valyrian steel, but the secrets of its making had been lost when the Doom came to old Valyria. "The colors are strange," he commented as he turned the blade in the sunlight. Most Valyrian steel was a grey so dark it looked almost black, as was true here as well. But blended into the folds was a red as deep as the grey. The two colors lapped over one another without ever touching, each ripple distinct, like waves of night and blood upon some steely shore. "How did you get this patterning? I've never seen anything like it."
"Nor I, my lord," said the armorer. "I confess, these colors were not what I intended, and I do not know that I could duplicate them. Your lord father had asked for the crimson of your House, and it was that color I set out to infuse into the metal. But Valyrian steel is stubborn. These old swords remember, it is said, and they do not change easily. I worked half a hundred spells and brightened the red time and time again, but always the color would darken, as if the blade was drinking the sun from it. And some folds would not take the red at all, as you can see. If my lords of Lannister are displeased, I will of course try again, as many times as you should require, but—"
A Storm of Swords, Tyrion IV