You can also find green stuff at most miniature companies however I would recommend doing some price shopping before you buy, compare the price to the length of strip you get. The product above is about a 2 foot strip (don't quote me on the length but I'm very close) for $12.00.
Green Stuff comes in a multicoloured strip (yellow and blue). The blue part is the hardener and the yellow part is the softener. When mixed in a 50% 50% ratio it will cure in about 20 minutes. Again if you use more hardener the putty will be thicker and harden faster; add more softener and it will have a thinner consistency and take longer to dry.
Green stuff has a very soft consistency to it. This characteristic makes it ideal for organic sculpting. It's great for things like faces, capes, hair, and other non technical objects. It is somewhat hard to master using green stuff (something I still haven't managed) but once you do it can hold very sharp features; just take a look at any of Tom Meier's work and you can see how truly amazing this putty can be in the hands of a master.
My only gripe about green stuff is that it can't be sanded or shaved once it dries. It is very unforgiving in this way. If you mess up a part or need to smooth something out you pretty much have to cut away the offending section and redo it.
Next up -Procreate