Ok so novice sewer here! I had a mini sewing machine as a teen and loved it. No idea what ever happened to it...

So, I was given my grandmother's overlocker and know NOTHING about them. My Nan isn't around anymore to ask so I thought the next best place is here

What's the difference between an overlocker and a sewing machine? Do you need both or just one? Thinking of starting to make some dresses for DD and some cushion covers etc to start with until I'm more confident. Should I go out and buy another sewing machine or will the overlocker do it?