In a broad sense, a processed food is a food that is not available naturally:
bread, corned beef, lollies, sugar, cheese, canned soup, etc.
There are a number of reasons food is processed, including preservation, presentation, taste, safety and aesthetics.
There may be some loss of nutritional value caused by the act of processing the food, but sometimes the processing will concentrate nutrients.For example, white wheat flour has had its wheatgerm removed, which contains, IIRC, about 30% of the wheat's nutrients. However, the advantage of white flour is that it keeps for years - if the germ remained then it would keep just a matter of months until the fats it contained went rancid.
Some natural foods are dangerous over long term use, such as soybeans, and have to be processed by fermentation to denature dangerous chemicals and proteins.
One good piece of advice is to always read food nutrition labels on supermarket goods. After a while you will quickly get an idea of what to avoid or limit.
When trying to lose weight it's generally best to avoid most processed foods. That's not because processed food is somehow wrong or bad in itself, but it's due to the commercial decisions the food companies have to make in order to get us to buy it in commercial quantities.
Food manufacturers will often add cheap taste enhancers such as salt, fat and sugar to foods even if they're not needed, simply because it's a cheap way to make it taste good. Those big three additives are added unnecessarily to a lot of processed foods (eg breads, soups) at the expense of more expensive nutritious components. Most of the excess salt and sugar in an average Australian diet comes from processed foods.
Which is why health-conscious people control what goes into the processed food they do eat by processing it at home: they make their own soups, simmer sauces, milk substitutes (soy milk, oat milk), breads, cakes and pastries (a nice healthy crumbly pastry can even be made from cauliflower!).
There are some articles on the website here - use the search box at the top right for more.
Here's one helpful article I found:http://www.weightloss.com.au/articles/h ... -loss.html
The food tables also list the fat content for common processed foods found here in Australia:http://www.weightloss.com.au/food-tables.html
Having written all that, I probably haven't quite answered your question the way you wanted, but it seemed like such a good topic to discuss I just couldn't help myself.