People use "black and white" loosely, but technically they're two different things.
Grayscale
Every pixel is a shade of gray, from black to white — typically 256 levels. This is what most "B&W photos" actually are. It keeps smooth tones but drops color data, so files are smaller than a color version.
True black-and-white (1-bit)
Only two values: pure black or pure white, no grays. Tiny files, but smooth photos turn into harsh dots (dithering fakes shades). Used for line art, scanned documents and fax.
Practical notes
- Grayscale is the right choice for a "black and white photo" look.
- A grayscale JPEG is smaller than color; learn why in color bit depth. To shrink any image further, compress it.