Stock is the water that vegetables, meat, or seafood have been cooked in. The items are then removed, usually discarded. The savoury water remaining some of the flavour and nutrients from the items making it more than just water -- it's now "Stock." Stock is used in sauces, soups, stews, and in many other things. It adds far more flavour and nutrition than just pouring in water would do, no matter how fancy the label on your water bottle.
It is surprising, though, the level of pretension and absurdity that usually accompanies writing about Stocks. Food writers will have you work two, three hours on a Stock before you even start on the recipe at hand. When we raised a minor rebellion several decades ago by using Stock cubes, the food writers slapped our hands and told us they would have none of that. Back to your Stock pots, all of you. And back we went.
The thinking about Stock espoused today is actually quite old: it dates from Pre-Revolutionary France, a time of great châteaux and great excesses, so great that the bankrupt economy collapsed into revolution and chaos. In these kitchens, the methods and costs of making ever more recherché variations of food to tempt the world-weary palates of the aristocracy was not a concern: there were kitchen staff whose job that day was just to make Stock; and if cost were ever calculated, it need not factor in the starving hoi polloi outside the gates whose taxes had paid for all this.
French-style Stocks, which is what we are all taught, have you boil up top quality food, then throw it away and keep only the Stock. This is both wasteful, in a world in which there has yet to be any such thing as "extra food", and time-consuming.
Instead, start keeping the Stock that you produce all the time in your kitchen anyway: when you tip your pot of your potatoes, carrots or corn into a colander in the sink, what is that rushing down the drain? That, gentle reader, is Stock. Gorgeous, abundant, nutritious, flavourful, cost and labour-free Stock that you have thrown away without a thought today, while tomorrow you will be reaching for a Stock cube and feeling guilty for not having real Stock -- such as that threw out the day before.
Here's how it works. You're about to drain a pot of veg. You grab a colander. Now, reach for a mixing bowl as well. Put the colander into the bowl, and drain your veg. Set the bowl with the Stock aside on the counter, and get on with putting dinner on the table. Comes time for kitchen clean-up, you open the freezer, grab out the large container in which you're freezing Stock, empty the bowl into it, place back in freezer. Bowl in dishwasher. End of story. It doesn't have to be any more complicated than that.
You may wish to get slightly more organized than that. You may wish to keep one large container, in which you pour mixed stocks, with both the mixture of stocks and the size of the ice block that will form in it being appropriate for large, hearty stews. You may wish to keep as well some smaller labelled containers, one for say potato stock, another for corn or peas, etc. You can even, if you want, freeze it in ice-cube trays, and then when frozen, tip them into freezer baggies and label them. (Though this makes calling on small amounts of stock very easy, it definitely would be classified as being very organized.)
Some Stocks, such as potato, may be cloudy when you freeze them owing to the vegetable starch in the water. Never mind: just freeze it as is straight off the potatoes. When the time comes that you need the Stock, if you're going to use it in some way that requires clarifying the Stock a bit, then when you thaw it strain it through a piece of paper towelling or a coffee filter. If you're going to use the potato Stock in a stew or in something where the starch in the Stock doesn't make a bean of difference, or where you in fact want the extra thickening power, then of course just use it as is, thawed.
Right then, let's go through this noting the possible objections.
1) It's too much work; I can't be bothered.
It's no work at all! Drain veg into a bowl, dump the Stock into a container and freeze. If that's work in your books, then you're probably not cooking veg to begin with to even get any Stock off of. Beyond that, it actually saves you work. You'll never have another emergency trip out your door at the last minute for broth cubes, and the tastes that you will get in your cooking will be so fabulous you won't be panic-frozen at crucial moments in the kitchen because what you made tastes thin.
2) It's not enough work; there will be no complexity of flavour.
Think this through. First, if you are using one of the large "mixed Stock" containers, there's going to be flavour complexity aplenty for your soup or risotto. Secondly, if you're using one of the smaller containers that holds just one type of Stock in it and you're worried about not being able to compensate in your dish for a complex Stock flavour, then why not thaw a second container of a different type of Stock, mix the two together, toss in a few bay leaves, cloves, etc, then zap for a few minutes in the micro or heat in a saucepan?
3) Thawing it is the part that sounds like it will take a lot of time.
Feh. Transfer the container from the freezer to the fridge the night before (which I rarely do, as I don't always know the day before that I need it); or put the container in a pan of very warm water for it to thaw around the edges for about 10 minutes, then dump the contents in a saucepan over gentle heat, to melt the central core that remains frozen. While that is happening get on with other parts of your meal. Or zap in microwave (if the container you froze in was microwave safe, and if you don't mind using the extra electricity.)
The truth of the matter is that elaborate French Stocks are so wasteful, and so much work to make, that we don't end up using Stock as much as we could in everyday cooking. Stock cubes from the store are very convenient, but there's always that background taste of a cube, isn't there, and so much salt? And with a cube, you've added flavour and some colour, but you've added no nutrition. Tinned Stock always tastes tinned -- it has all that sodium in it, and it really pushes up the cost of what you are making.
This alternative strategy of "rescued Stock" will revolutionize how what you cook tastes. In many places where you used to use just water, you'll use Stock instead. You'll save money over buying commercial instant-ready substitutes such as cubes and tins. And it's all incredibly common-sense: you already have the flavour, the nutrients: why would you cook veg one day, through out the Stock, and the next day, cook veg, throw them out and keep the Stock??
When making any Stock that you are planning to freeze for future use, it's best to season and flavour it only lightly, if at all. The less specific flavour you add now, the more general purpose it will be -- fancy up the flavour when you use it in a recipe. Too highly seasoned a Stock could overpower what you make with it.
You may wish to keep some cheapo plastic ice cube trays to use for freezing food items in. You can freeze Stock cubes in them, then when frozen tip them into bags to free the ice cube trays for other uses (as well as to prevent someone mistakenly popping out a few chicken Stock ice cubes into a batch of martinis. ugh.) Freezing in small quantities like that allows you to draw on small amounts of Stock as needed: 1 ice cube equals 1 tablespoon.