Accorrding to Wikipedia:
A terrace is an outdoor, occupiable extension of a building above ground level. Although its physical characteristics may vary to a great degree, a terrace will generally be larger than a balcony and will have an "open-top" facing the sky.
Terraces are highly versatile in terms of functionality and can be used for a variety of activities including, but not limited to: gardening, relaxation, entertaining guests, sunbathing, and barbecuing. Sometimes, there is a hot tub, jacuzzi, or a small swimming pool on terraces.
There may also be an outdoor kitchen or a barbecue grill. Extra space adds value to the property.
Well - according to Skyhatt a terrace is this:
And in August we are going to fill it with beer and burgers. Welcome :)