How to use waypoints which have been collected by GPS for mapping?

You can load gpx files with waypoints in nearly every OSM editor (that is JOSM, iD and Potlatch2) directly from your local PC (without first uploading to OSM.org). They then also show waypoints. For iD: just drag a gpx file onto the editor map; for Potlatch2: Menu "background" → "Vector file" → "Open".

In addition uploading gpx files with waypoints to the osm.org server is possible - but they also need to contain tracks (one track point with a time stamp is enough). As far as I know the waypoints are not available publicly but for yourself – at least Potlatch2 (loading your own traces via the gps menu) then also shows the waypoints with labels. There is also this slightly outdated how-to for waypoints with Potlatch2. Upload_GPS_tracks#Uploading_Waypoints links to there.

To just view gpx files

• you could use JOSM, iD or Potlatch2 with a mapnik background (no vector/data objects – iD: Menu "background settings" → "background" → "OpenStreetMap (Standard)"; Potlatch2: Menu "background" → "Vector file" → don't show "main").
• you could also upload your gpx file to a website (some offer simple temporary upload without registration) to view it: see Track drawing websites in our docu wiki

Yes, I'm pretty sure the "upload gpx" feature on the main website only accepts gpx with tracks.

The JOSM editor allows you to load gpx files containing both tracks and waypoints. Normally on the eTrex, there will be two different files: one for the waypoints and one for the tracks.

There are many options to view your points :

• Mapsource can definitely show a basemap, but I dont know it well enough to explain how to do it.
• For most desktop map and gpx viewing, I recomend marble which is light, simple , and versatile. It comes with some default osm maps but it is easy to add any other slippymap.
• Altough it may be overkill, JOSM can be a great way to view gpx files.
• Lastly, if you're ok with a little bit of coding, using a slippymap such as leaflet or openlayers (by creating a local html file) is another versatile way to display gpx data.

Is it correct that OSM only uses tracks?

No, as has already been mentioned, most OSM editors are capable of processing GPX files containing both GPS tracks and waypoints, but in order to upload a GPX file to OSM there must be at least a small GPS track in the file as well. This track is an example that I edited manually ages ago to show that it can be done.

Also, is there any way to salvage the marks that we collected and turn them into usable data points?

Yes, if you fake a short section of GPS track in the file with waypoints in it. If you download the example that I mentioned above and look at it in a text editor, you'll see what needs to be added.

If you find yourself wanting to work with GPS traces a lot, you might find GPSBabel useful.

If so, how can I transfer this data to my PC and visualize the marks on a map? I tried using MapSource, but upon loading there is no map image in the background.

It is possible to load OSM data into Mapsource, just as you can into Garmin devices using the output from mkgmap. If you're doing it for the first time, I'd suggest looking at http://garmin.openstreetmap.nl/ (there's an FAQ here).

Finally - a couple of notes about uploaded Waypoint support in OSM editors. If you're using JOSM, you might want to have a read of this previous question and also this one and this one. This one asks about Potlatch 2. Potlatch 1 contains an extra useful "waypoint" feature - you can move unlocked items around, which makes placement easier, although because it lacks support for e.g. Bing imagery it can be difficult to work in just that.

In iD I can actually simply drag and drop either the tracks or the waypoints directly from my garmin etrex unit connected as a usb mass storage device. However I cant get them both to appear at the same time. (if theres a simple way to do that it would be convenient).

If I have the track showing and I drag and drop the waypoints of a particualr day or session, the track dissapears and shows only the waypoints.

I usually edit with potlatch2 and under the Options tab tic show mouse pointer lat and lon. It displays in degrees and decimal degrees. Most hiking Garmins, the etrex included, can also be set to this format so you could transfer them manually. You may get better accuracy with a trace, more readings so a better average. Some times if i want to mark a crossing or small feature I will walk up and down a few metrers or arouund it so it can be seen in the trace. ( for best results set to record once a second if you have a micro SD installed). Some garmins can average a mark ( the Vista HCX does and I think yours can to) if yours does you could try that next time.