Welcome to Wikipedia users
Welcome to OpenStreetMap!
We're always pleased when new mappers join our ranks and Wikipedians are as welcome as everyone else.
OpenStreetMap (aka OSM) is 100% independent. It is not a Wikimedia Foundation project, but it is supported by its own foundation. Although OSM is also a collaboratively edited, openly licensed compilation of knowledge, we are very different from Wikipedia in many ways - the rules and community norms you have learned at Wikipedia don't apply here.
This page aims to help you on your way by summarising the key differences between the two projects' philosophies.
Original research always wins
The fundamental tenet of Wikipedia is "no original research". We're the opposite. In OSM, the gold standard is original research - or as we call it, surveying.
Going out and surveying roads, paths, shops and landuse makes OSM what it is. The best edits are those which have been researched on the ground. We always defer to local knowledge.
That applies equally to the name of your local shop and to disputed territories: we record the names that are used in a particular area, no matter what individual governments might say. (See this guidance note.)
Because surveying is paramount, you should be very wary of "tidying" or "gardening" OSM data in the way that you might be used to with Wikipedia. A well-meaning change to unify data might remove nuances that the original surveyor recorded.
However, there is one principle that is similar in OSM and Wikipedia: NPOV, aka Neutral Point Of View. In fact, we go further: No Point Of View. We only record facts, not opinions, even measured ones. OSM isn't the place for subjective quality judgements.
We are much stricter about copyright
You must not copy anything from other maps or images into OSM. Not Google Maps, not Google Street View, not a paper map. The only third-party sources we admit are those which are out-of-copyright (see e.g. outofcopyright.eu for Europe); those which have been explicitly dedicated to the public domain (e.g. US federal and PRC national publications); and those where OSM has formal permission (e.g. Bing aerial imagery).
Some editions of Wikipedia allow certain media to be uploaded under "fair use". We don't. Even if the copyright laws of your country might permit such a use, OSM aims to be usable worldwide. We're not here to explore grey areas, we're here to make an awesome map that everyone can use.
Wikipedia is legally based and largely hosted in the US, while OpenStreetMap is legally based and largely hosted in England. UK copyright laws are generally stricter than US ones anyway. Even UK fair dealing exceptions are not admissible for OSM use.
We don't have a notability rule
That said, if you want to cite your sources, go for it; we have things like photo linking and key:source to help with that (not to be confused with key:ref, which isn't short for 'references' like the section at the bottom of Wikipedia pages, but instead the referer-referent relationship -- that is, identifiers). Just recognise that you are assisting other mappers (and also downstream users) to verify a bit of mapping, you're not 'defending' it against alterations.
Talk pages aren't the primary means of communication
You're probably used to communicating with other Wikipedians via talk pages.
Though OSM does have a wiki (this one), it's only one of the community's tools, and many OSMers don't use it. Most general discussion happens on mailing lists or forums, while mapper-to-mapper communication happens on the main OSM website via messages, changeset discussions, and diary postings, sometimes also face to face at local meetings. If you want to talk to OSM wiki users about the wiki, by all means use their talk pages, but don't expect others to watch such pages.
Your edits show up everywhere
Just as Wikipedia content is freely reusable, OpenStreetMap data is used in thousands of different places - from Apple Maps to printed cartography. openstreetmap.org is just one view on the data, intended primarily for mappers and enthusiasts of geography.
So you shouldn't change things just for one use. If (say) a city name is missing from Wikipedia's OSM-derived maps, don't move the city around in OSM so it shows up in the place you want - because that might break another use. Rather, you should aim for the data to be objectively correct, and let the map-drawing programs ("renderers") sort out the details for themselves.
We don't have admins or other super-users
OSM does not have administrators (admins) or super-users. We don't lock map data. We don't have a semi-protection measure. We do have a small volunteer Data Working Group who are able to rule in cases of dispute and block users, but that's very much a last resort.
You don't need to categorise everything
Wikipedia articles are often organised by category. We don't do that for map data. Because OSM is a spatial database, anyone can take the data and run a query to give (say) "all convenience stores in Germany". So you don't need to create a "German convenience stores" category: that information is already explicit in OSM. (See Relations are not categories for more.)
We don't have persistent IDs
Objects in OSM have an ID and can be accessed through a URL constructed from that ID, e.g. https://www.openstreetmap.org/node/2817389220. While these IDs don't change without reason, there are situations where they will change. Unlike Wikidata (where IDs are stable) or Wikipedia (where renaming an article will usually at least leave a redirect behind), OpenStreetMap doesn't have IDs that are guaranteed to be permanent.