Not all solders come with flux inside. As you see here we've got two samples of different types of solder. One is rosin core, which has the flux inside of a sheath of solder. On the left we have solid solder which has no rosin flux at all. Some natural questions come up: which one is appropriate for the work that I'm doing, is the flux that I have enough for the job, do I need to add additional flux, and so forth.
The answer to that depends on what you're working with. When soldering electronics the electronic printed circuit boards begin with a nice clean metal surface. There's very little oxide to remove. The parts that go on the board are also clean. It tends to be that the rosin core solder holds more than enough flux to clean the oxide from the parts as they're soldered.
However, in the case of other parts, such as copper wire, stranded copper wire has a great deal more surface area which oxides could form on. So there's a lot more material to remove and chemically react with. In these cases rosin core solder might not be enough. It's usually a good idea to apply additional flux outside of the rosin core solder.
Now, you may run into different types of rosin core solder. The most common are perhaps the single core, such as this, which has a single cylinder of flux in the center of the solder. However, more advanced types of solder may have more than one core. In these cases they may deliver a little bit more flux, and you may be able to work with parts that are a little more dirty. They may deliver it more evenly. In general, that's a specialized type of solder which is not really that common. You're most likely to run into the single rosin core solder such as this.