When you crop an image you inevitably lose resolution, as you delete parts of the recorded image to better frame the remaining. This will become more noticeable when you enlarge the resulting image on a large screen or print, depending on the final resolution.
But beside that, you may lose some more detail due to a number of factors:
- saving the jpeg with lower quality settings than before;
- downscaling the resulting image further;
- using a software with poor jpeg compression;
- cropping (or any editing involving saving and re-loading the picture) multiple times may result in repeated resampling of the image;
- cropping to a size which is not a multiple of the block size of the jpeg may also result in resampling and loss of detail.
Pushing the conversion from RAW to JPEG as late as possible in the editing process will surely preserve the resolution better, but if you take care of these aspects you will get very similar results with the JPEG as well.