What is the main difference between depression and schizoid personality disorder?

Big difference: While schizoid personality refers to a relatively set way of integrating information, processing and outward behaviors (i.e. Personality), depression refers to very specific symptoms that cause dysfunction in daily life. These symptoms can affect sleep, eating, energy, interest in life. While people with schizoid personality can get depressed, they are usually not affected by their traits.

Mood: Depression is a mood disorder involving sadness, emptiness, withdrawal, and anhedonia, among other symptoms. Schizoid PD is all about indifference to interpersonal relationships.