In a very broad terminology:
I understand Application Layer Denial of Service (ALDoS) to be a 1 machine on 1 machine attack.
I understand Distributed Denial of Service DDoS (DDoS) to be a 2 or more machines on one machine attack (usually a thousand machines attack one machine).
If we go broad enough, even operating systems are computer applications.
These are my conclusions as a non IS expert who tried to read and listen to conversations. I might misunderstood anything but I just want to know if this broad distinction is enough accurate.