I've been there.
It's a huge stone building, with a dull gray metal dome. There is intricate stonework, in places, but it's overshadowed by the nearby Dome of the Rock.
Tourists and pilgrims line up outside, and faithful guards man the doorways -- cameras and flash photography and shoes are not allowed.
Stocking footed, silenced, I passed the bored-looking guard, and entered. Hand-woven silk rugs lined the floors, demonstrating the opulence and elegance the Mosque had to spare. A worker stood on a ladder on the other side of a roped-off row of columns (two rows of gray-green columns ran the length of the building), painstakingly painting the Arabesque loops and whorls of a design.
Inside, besides the columns and painted designs, and clumps of hushed tourists, is... nothing.
Yes, at the very back of the building (or the front, or the north) is another prohibited area. There's a tall but narrow wooden box, a temporary room of some sort perhaps. There's a chair. There's a microphone and a loudspeaker for the call to prayer.
And that's all.
The guide said that it was erected as a monument to Mohammed's spiritual journey on a winged horse (ala, Pegasus, if you are more familiar with Western mythology.) In the story, Mohammed takes a spiritual journey to Al-Aqsa (in Arabic, literally "A far away place.")
One of the three men fighting to become his successor came to Jerusalem, decided that the economic and political benefit of controlling a pilgrimage site was too good to pass up, and declared that the spot where Mohammed and his horse alighted was just to the east of the spot where he would build the mosque -- right atop one of the walls into the old Jewish temple compound.
That's the story I was told. I'm sure there are many stories.