There are 2 options available for sealing the floors - varnish or oil. First I should say they both come naturally in 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 goldenish colour.
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 product, 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 specialists flooring shops) would be Bona Traffic and Junckers HP Commercial. 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 in any time. Disadvantage is their long drying time (4-7 hours per coat). People however tend to miss that point, which ends up not in favour for the floors. I can recommend Osmo Polyx hardwax-oil products and 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, make sure you you have protected the floors well until you apply 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 application of first coat and before application of last coat.
Hope that helps.