The dandelion is one of the most common lawn weeds. It is a perennial broadleaf and it can spread easily because it generates seed puff balls that float in the wind long distances. If you have a lot of dandelions you’ll want to apply a herbicide like 2,4-D. It’s fairly easy to kill them.
An alternative is to hand pull them. There are some weed pulling tools available that make it a bit faster, but I personally just use a screwdriver. I’ll stick it into the ground following the taproot and use it to pry it up. If you go this method, it’s important to get the full taproot or it’ll just grow back quickly.
Having a thick, healthy lawn also helps prevent them. As my lawn went from ugly lawn to healthy lawn, each year the number of dandelions popping up shrunk. Even with a neighbor whose yard is filled with them, my lawn stays free of them using these techniques. So if you have them, don’t blame your neighbor – kill them, and prevent them!