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mollie m.,
It could be a type of cherry tree. I have some of these in one corner area of my garden. They are fast growing and can be "spindly" as they go for height rather than density. Also, the trunk slowly thickens as much of the energy is put into the height of the tree; this is possibly due to the tree being able to bend quite a lot without breaking + the distance between branches - and, also, the design of the leaf.
These are the basic details of my wild cherry trees: tall and slender, small white flowers in Spring, small red berries/cherries (which the birds love), slim trunk in comparison to the height, silverish grey-brown bark, quite rough bark which has rough rings (one-after-another) all up the trunk etc.
Check this out: Prunus avium
Also, you could check out Wild Cherry trees.

Good Luck!