Both words can be translated as some. But that does NOT mean that they are interchangeable. Here is how to use them correctly.
| Use некоторые, followed by the nominative, unless inflected by case, when you are referring to some out of a whole group of items, people, etc. |
Here’s a really good sentence to help you clearly see the difference.
| Некоторые были против нового закона, а другие – за него. Some (of a group) were against the new law, while others were against it. |
Another example below:
| Из всех учеников только некоторых поставили пятёрку. Out of all the students only some got an A. |
| Use несколько, followed by the genitive, when you are not referring to a part of a whole group. |
| Я получил несколько писем от иностранных друзей. I received some letters from foreign friends. |
| В очереди стояло несколько человек. There were a couple people in line. |

