Alternate - refers to the position of the leaves on a stem or branch. Leaves are not found opposite each other, rather one on each side as one moves up or down the stem. This feature helps in plant identification.
Compound - refers to a leaf which is divided into leaflets. This can be easy to see as in the case of low plants such as the pea family, for example. However one can be more easily fooled in the case of trees, such as ash, Here one might think that each leaflet is a leaf. One way to tell is by noting that such "leaves" are always appearing as a set of 7 or 9 or some other number, on the branch, This only occurs as leaflets of a leaf, not leaves on a branch.
Opposite - refers to the position of leaves on a stem or branch. The leaves appear as a pair, one on each side, precisely opposite each other, This feature helps in plant identification,