People often confuse two classes of evil beings in Tolkien's writings, "wraiths" - most prominently the Nazgûl or Ringwraiths - and "wights" - most prominently the Barrow Wights. Here is some information to clear this up.
Wraiths
The basic difference between those two is that wraiths are invisible. This is proven without doubt by the example of the Witch-king, the lord of the Nazgûl: "The Black Rider flung back his hood, and behold! he had a kingly crown; and yet upon no head visible was it set. The red fires shone between it and the mantled shoulders vast and dark. From a mouth unseen there came a deadly laughter." (LotR, The Siege of Gondor).
However, we also know that this invisibility was not perfect. In dim light, on a cloudy day, it concealed the wraith entirely; but under direct sunlight, he was faintly visible as a shadow. This is exemplified when Bilbo - wearing the One Ring and therefore invisible - wanted to escape from the orc-cave: "(...) the sun came out from behind a cloud and shone bright on the outside of the door - but he could not get through. Suddenly one of the goblins inside shouted: 'There is a shadow by the door. Something is outside!' " (The Hobbit, Riddles in the Dark). We can safely assume that a Hobbit wearing the One Ring showed characteristics comparable to a common wraith.
Wights
Barrow-wights, on the contrary, are clearly visible. Frodo was able to see one - or rather, his arm - even in a rather dark tomb: "Round the corner a long arm was groping, walking on its fingers towards Sam, who was lying nearest" (LotR, Fog on the Barrow-downs) This proves their visibility beyond doubt. I think they are more like mummies than ghosts, rather shrivelled and dried. And we can deduce that they have at least one exceptionally long arm (see Fig.1).
I have also been asked why just the arm was creeping around the corner, and why the hobbits' clothes could not be found once they were rescued by Tom Bombadil. These questions are connected, and the explanation is simple. The wight had spent some centuries in a grave, and his own clothes had rotted away. In other words, he was stark raving naked. So he was hiding behind that corner, feeling too embarrassed to come forth. He had stolen the hobbits' clothes for himself. (He was unable to put them on though, as they were much too small for him.)
The origin of the term "Wight"
One can ask now why Tolkien, when translating LotR from the original Westron, introduced the term "wight" for this sort of barrow-dweller.
The truth is, this was a little joke. A "wight", in English, is of course someone from the Isle of Wight. This is the sunniest part of England, so Tolkien suggested that the people living there - and the tourists flocking there - were exceptionally tanned, shrivelled and mummy-like. And people say Tolkien had no sense of humour!