How to imagine a 4 dimensional universe:
One day Mr A Square woke up.
He lived his entire life in "flatland". To you , it would look like the surface of a table.
All Mr A Square could see was things on the surface of the table: to him, space was two dimensional. He couldn't see within the body of another Square, he could just see the perimiter.
Then, along came Mrs ThreeDee. She started talking with Mr A Square, by drawing two dimensional images onto the table. This was very strange to Mr A Square, things appearing out of nowehere, and disembodied voices talking to him. Mrs ThreeDee taught Mr A Square all about the third dimesion, or tried to, and showed him how she could reach inside objects and modify them or move items through the 3rd dimension and appear to teleport them.
(the above based very roughtly off Flatland:
http://www.amazon.com/exec/obidos/tg...76162?v=glance )
Ok, now imagine if Mr A Square didn't live on a table, but rather lived in a cube. At any one height, he could still only see two dimensions, but going up or down other "times" would exist, outside of Mr A Square's perception.
A moving slice through this cube would be completely indistinguishable from a moment of the table-top flatland.
Now, imagine if you are living in a 4 dimensional universe. The extra dimension is time.
There is reason to believe it is worse than that: remember that in the "cube" flatland, the time dimension was measured by height: what we percieve as time didn't really matter to the cube, because all times (heights) existed at once.
If the cube blinked into existance for a fraction of a second, and then disappeared, it would be the same existance to residents of Flatland as if the table top existed for a long time.
Physics is indicating there may be something like 10 dimensions out there. 6 of them are very small, so small that we don't notice them, 3 of them are space-like and one of them is time-like.
That was too much blather. =)