both have good environmental and financial credentials.
The first kind are to use in the washing machine, instead of cleaning liquids or powders – this also means they are good for those with sensitive skin. There are lots of versions of these, some egg shaped, and they come in different sizes offering a variety of number of loads of washing so do your maths to work out which is best per wash.
The second sort of laundry ball is for the tumble dryer (we know they are evil, see our separate post!) they are to use instead of fabric softener, and to reduce the drying time needed. Again, there are varying colours and sizes available, but these tend to last forever.