@donnellysdogs There are a number of different varieties of avocado, but in the UK most of them are very hard to find. The most easily available nice one is the Hass avocado. They're usually labelled as such. They have a good oil content and are creamy/buttery and nutty. Often they're the more expensives ones in a supermarket if the supermarket only has two kinds. As with everything, you get what you pay for.
There are lots of varieties of olives. If I were you, I'd go to a nice deli or the deli counter of a supermarket and taste a few. Yes, there is a difference in taste between black and green, but that difference varies depending on the variety. Generally, green ones are 'fruitier'. Black ones can be 'meatier' or even slightly smoky.
A good black olive is the Kalamata from Greece (pretty widely available - you can get them in a jar, if no luck at a deli). For green olives, I like Cerignol and Gordal. They're both quite big and are sometimes stuffed with things like lemon, garlic, etc.
Black olives are basically ripened green olives, but many of the 'black' olives sold cheaply in jars or tins have actually been ripened with gas and turned black unnaturally. They're horrible, in my opinion, and have a horrible texture.
There isn't really a difference in taste between stoned and unstoned. But lots of the premium ones have their stones left in whereas the cheapies are often stoned. If you see a jar of stoned black olives for 80p, they've probably been artificially blackened and will taste either bland or bitter. Again, you get what you pay for. Choose a premium smaller jar of, say, Kalamata, or get some from a deli. Even the ones that come in plastic pots with herbs and feta, etc, wil be better than jarred or tinned olives.
You can use them in cooking as well as salads. I usually buy cheaper but ok ones if I'm going to cook with them, but more expensive ones for a salad.
I sound like I work for the Olive Marketing Board! I don't : D I just like olives and olive oil - and I think they have big health benefits
Hope that helps.