There are 2 options available for sealing the floors - varnish or oil. First I should say they both come naturally in the clear finish but may come in different sheens - matt, satin, gloss. Both of the products add a bit to the colour of the wood, e.g. pine and oak will go in a more goldish colour.
The varnish is a polyurethane product, which is either water-based or solvent-based. Water-based products, have a low odour, quick drying time and are easy to work with. They are durable but thinner products (3 coats recommended) than solvent-based products, which nowadays are less popular due to the strong odour and the slow drying time (can go to 12 hours per coat).
Some of the most durable water-based products you can find (usually in specialist flooring shops) would be Bona Traffic and Junckers HT700. These are clear finishes, which come in matt or satin sheens. These are two-component products, which are mixed before application and once mixed they should be used within 3-5 hours. These products are designed to withstand shoe traffic.
Oil-based products are not that strong sealant compared to varnish in the long term, but they offer the option for recoating at any time. The disadvantage is their long drying time (4-7 hours per coat). People, however, tend to miss that point, which ends up not in favour of the floors. I can recommend Osmo Polyx hardwax oil products, especially the new Osmo Pure 2K, which is a quick-drying product for hardwearing use. It can be recoated as all other oil-based products.
If you have finished with the parquet floor sanding, ensure you have protected the floors well until applying the sealant, as you may end up with stains, which will come out after the application.
It is recommended you do a fine sanding or buffing with a light abrasive before the application of the first coat and before the application of the last coat.
Hope that helps.