If you need to remove the cap in order to write with the pen, it is either a fountain or a rollerball pen. Rollerball pens have a liquid ink refill.
If you twist or push the back end of the pen to advance the refill to write, this is a ballpoint pen.
Rollerball and ballpoint pens are not interchangeable. The design of the two pens are different and will only allow for the refill that is specific to that pen type.