Hi Yara,
Both provide a high end workflow platform and forms engine that works well with SharePoint as is far beyond what you can do natively in SharePoint without a lot of custom coding.
K2 has evolved from a standalone workflow and data modelling engine, i.e. it is not dependent on SharePoint and is capable of creating worfklows and forms around business data that comes from SQL databases, Active Directory, SalesForce or many other sources, but it does have very good SharePoint workflow, forms and automation tools in it's current iteration.
Nintex has evolved from a more SharePoint-centric platform that strived for ease of use and a laser focus on SharePoint to in it's current iteration having connectors for different data and being more well rounded than earlier iterations.
In their current forms they have evolved to being more similar to each other (IMO) with K2 having improved ease of use and Nintex having more non-SharePoint options, depending on on-premise or hosted options. Note that both have changed rapidly in the last year or two particularly to keep up with the move to the cloud and Office 365/ hybrid SharePoint, so I recommend looking for more recent comments about their current iterations.
