Oaks
To the left, you can see leaves from three different types of oak trees. The Pin Oak, Quercus palustris, is found in the northern part of the U.S. and southern Canada, in lowland areas with moderate shade. Its leaves are usually 3" to 4" with 5 lobes and a wedge-shaped base. The Jack Oak, Quercus ellipsoidalis, is found on higher ground and its lower branches are usually dead. It has dark green leaves and the tree has an overall untidy look. The Scarlet Oak, Quercus coccinea, grows in sandy or gravelly areas.  Looking at its leaves you will find a thin, delicate blade with lobes that sometimes almost enclose oval spaces. The mid-rib of the leaf is yellow. 
 

Walnuts
To the right you can see two leaves from walnut trees. Leaflets arranged along opposite sides of the stem like these are called pinnate compound leaves (for more information, see Leaf Terms). The Black Walnut, Juglans nigra, is found on high ground over a vast majority of the United States. It is somtimes missing the end leaflet, and all the side leaflets are attached to the stem at an angle. The Butternut White Walnut, Juglans cinerea, has an end leaflet, and its side leaflets are attached to the stem perpendicularly.