For this moment in time, this won't be pretty, I just need to get this out.

Okay so, seeing the Creevey family on another site be regarded to as pureblooded just annoyed the ever living shit out of me. Also, sometimes people forget how many ways you can be defined as halfblooded. At the end of this I'll link you to HP Lexicon and I suggest that before making a canon, to just check to make sure which of your character's parents were created by JK (a lot of them are actually) and whether or not they have a set blood status. Think of blood status like wealth to us "muggles". Halfbloods are usually middle class, squibs are usually "dirt poor," muggleborns are usually lower middle class and maybe even poor, and purebloods are usually wealthy and powerful or high upper class. Just to give you an easily understood analogy

Purebloods - This is the easiest one to figure out.

Both parents have pureblooded parents and no muggle blood can be traced back to their great-grandparents or great-great-grandparents.

Usually extremists like the Malfoy's and the Black's will simply erase squibs from their family lineage as well as any members who marry muggles, muggleborns, or halfbloods. But because purebloods need to mate with these people they so often try to avoid to keep from dying out, there is a lot of inbreeding in families, like with our muggle royalty. And because of this, families that are pureblood extremists like this are more likely to give birth to squibs, who have magical lineage but have no magical abilities themselves. Or these families are more likely to give birth to only one child because there is probably complications in the wife's pregnancy - pregnancies if they keep trying and trying for that "male heir." Simple right?

Halfbloods - This list is a bit lengthier since there are many possibilities.

1 pureblood parent and 1 muggle parent
1 pureblood parent and 1 squib parent
1 pureblood parent and 1 muggleborn parent
1 pureblood parent and 1 halfblood parent
1 pureblood parent and 1 half-breed parent (can only really happen with giants, werewolves, and veelas; vampires cannot reproduce)
1 halfblood parent and 1 squib parent
1 halfblood parent and 1 muggle parent
1 halfblood parent and 1 muggleborn parent
1 halfblood parent and 1 half-breed parent
2 halfblood parents
2 muggleborn parents
1 muggleborn parent and 1 muggle parent
1 muggleborn parent and 1 squib parent
1 muggleborn parent and 1 half-breed parent

For the most part, this is how this is. Halfbloods aren't just about muggleborns and muggles. A squib is a nonmagical civilian (although they recognize magical auras and shit like that; remember Mrs. Figg and her ability to feel the despair dementors bring?) just like a muggle is and no matter who a squib's parents are, that just all goes out the window. Some halfbreeds are pureblood, but they are usually identified by their magical blood, whether it be lycanthropy, giantism, or veela blood. Vampires cannot produce, only turn/change.

Muggleborn - This is really simple actually, just like with purebloods.

2 muggle parents (most common)
1 muggle parent and 1 squib parent

There you go. That's all there is to it. We all know that two muggles who end up having a kid who gets a Hogwarts create a muggleborn. It isn't touched upon with squibs, but for the most part, squibs are often treated like muggles because they don't possess any magical blood even though they were born to two magical parents.

Okay, so there you go! Like I had promised, HERE is the link to HP-Lexicon's list of wizards and witches JK named in the series. You might find out your character's parents' blood-purity, but you might not. But I'm not changing any families to pureblood on the list. I can't stand that because again, purebloods are dying out! And just ask me whether or not I actually got the name of one of the parents off the Lexicon. Most of the family names are names that have been mentioned by JK in the books or in interviews, but it depends. And like, half of the mother's are canon names, except for most of the muggleborn families! It just bothers me a bit when people make every parent a muggle when it's known they went to Hogwarts ...

Thanks for reading.