A Blood knot is a good knot to use when tying two lines together that are about the same size. It's easy to tie but leaves a fairly big knot where the lines join. Fly-fishermen and saltwater fishermen use them to tie leaders together. This knot is time tested and is a very strong way to tie lines together.
When to Use the Blood Knot
Use a Blood knot any time you need to tie two lines of similar size together. They're also good for tying long fly-fishing leaders to different test lines, and making them fall straight. They are small enough to go through guides without much friction. Blood knots are also good to tie leaders in saltwater fishing when attaching lighter leaders to the main line.
How to Tie the Blood Knot
To Tie a Blood knot:
Start a Blood knot by overlapping the ends of the two lines you are joining together.
Twist one around the other at least five turns and then bring the end back between the two in the loop formed by the twist.
Do the same with the end of the other line, going in the opposite direction; be sure to make the same number of turns with the second line as you made with the first line.
Slowly pull the two lines in opposite directions and tighten the knot; watch to make sure the wraps slide together without lapping over each other.
Pull it tight and clip the tag ends close to the knot.
Tying a Blood knot.
A finished Blood knot.
There are many more knots you can learn to tie, and many of them have special purposes. Sometimes it's fun just learning to tie a new knot and seeing how it works for you. Experiment with several and pick what does the best job for your fishing.