Colin - blacksheep has explained what terrazzo is, and I would simply qualify this by saying I generally treat it as a marble floor. What can happen with a cement terrazzo is the cement wears away over time and dirt fills the holes that remain - the solution is to
1. Thorough deep clean and scrub, but this may involve harsh cleansers that could damage the remaining marble chips
2. Regrind the floor to a honed or polished finish - this will leave the floor smooth once more giving dirt nowhere to hide.
I would go for 2 as it is better in the long term.
As for sealing - well, for a cement terrazzo, if you think the floor will be affected by spills then apply an impregnator. If it is simply a walkway then don't bother - a sealer will not help to keep the floor clean!
Hope this helps