Blackburnian Warblers are found at the tops of trees, Canada Warblers lower down. Nuthatches, Brown Creepers, and Woodpeckers are typically found on the trunks and larger branches. Other definitions are used, but they are similar. A shrub or bush, on the other hand, is a woody plant having multiple stems growing from the same roots and is usually lower growing. An oak is a tree.
Most lilacs are shrubs.
At the other end, trees can be gigantic. Some species, like Giant Sequoias, Coast Redwoods, Douglasfirs, and some Eucalyptus, grow 300 feet tall or more and have trunks 30 feet across. Some species also have very long life spans. The Great Basin Bristlecone Pine often lives longer than 4,000 years. That's right, four thousand.