I withdrew some bitcoin from a service's account I maintain. The transaction shows up in my wallet, but it has been marked as "unspent" ever since. What does this mean?
When someone sends you bitcoins, they do so by submitting a Bitcoin transaction to the network. Transactions specify which coins they spend in the inputs, and allocate the funds to new owners on the output side. You received one such transaction output. While the other output got used already, your output is still marked as "unspent". "Unspent" in this context means "not spent yet" in the sense that it is still available to be spent. When you want to spend your bitcoins, you can use your wallet to create a transaction to send some or all of them to another Bitcoin address.