Hemp and Marijuana differ in 4 main ways: chemical composition, cultivation, usage and legality.

Chemical Composition:

Both hemp and marijuana are able to produce high levels of CBD but different levels of THC are produced.
Hemp: less than 0.3% THC
Marijuana: more than 0.3% THC (up to 30%)

Cultivation:

Marijuana needs lots of love and care and is grown in environments that carefully control the temperature, lighting and humidity.  The aim is to grow the female plants so the budding flowers develop.
Hemp is a crop that is often grown outside.  The aim is to maximise growth and yield and so the plant needs less care and attention.

usage:

Hemp can be used for food, clothing, cosmetics, shoes, textiles and rope, paper, building materials, wall construction, insulation, bioplastic and biofuel.  It can also produce CBD oil.
Marijuana is used recreationally and medicinally and the levels of THC means it binds to CB1 and CB2 receptors in our endocannabinoid system, resulting in the feeling of being ‘high’.  Marijuana can also produce CBD oil.

Legality:

Marijuana is not legal in the UK.  Even if CBD oil from marijuana plants contains less than 0.3% THC, it is still not considered legal in the UK, Europe or America.
Hemp-derived CBD oil is legal in the UK, Europe and America as long as it contains less than 0.3% THC.