Solid Hardwood is traditionally 3/4" thick, and comes in various widths, from as narrow as 2-1/4" up to 4" or 5". A sand-in-place hardwood floor comes from the factory milled and ready to be installed, but needs to be sanded and finished on site, generally requiring several coats (and a week or two). Pre-finished hardwood comes from the factory completely finished and can be walked on immedietley after installation.
The changes in surface wear layer technology has given rise to more and more pre-finished hardwood options in the past decade. The finishes are now durable enough that if you desire to refinish your pre-finished hardwood floors, it is recommended to only sand down the clear coat a little and re-apply it rather than try to sand through the entire factory finish.
Pre-finished styles range from a tradition smooth finish with eased edges to a more rustic wirebrush finish or hand-scraped finish with scooped edges or beveled edges. Proper professional installation insures the best performance (and fit and finish) and keeps factory warranties on the wood intact.