Main Similarity and Difference
'Something' and 'nothing' are both examples of indefinite pronouns.
They can be considered opposite, meaning that 'something' refers to a thing that is unknown or a thing that we don't know what it is.
On the other hand, 'nothing' means 'not anything'. It shows the lack of things.
As you can see in the examples above, these two sentences have completely opposite meanings because of the use of 'something' and 'nothing'.
Now, look at another example:
As pronouns, the difference between 'something' and 'nothing' is that something is an uncertain or unspecified thing, while 'nothing' is not any thing (no thing).