You gain the service of a familiar and the benefit that goes with it. The familiar appears in an unoccupied space within range and has the statistics of the chosen form, though it is a celestial, fey, or fiend (your choice) instead of a beast. The familiar's Intelligence Score is raised to 6. The familiar’s total hit points will always equal one-half of your own total hit points, rounded down.
Your familiar acts independently of you, but it always obeys your commands. In combat, it rolls its own initiative and acts on its own turn. It uses your Proficiency Bonus instead of its own. It also adds its proficiency bonus to its AC. The familiar is proficient in all saving throws.
While your familiar is within 100 feet of you, you can communicate with it telepathically. Additionally, as an action, you can see through your familiar's eyes and hear what it hears until the start of your next turn, gaining the benefits of any special senses that the familiar has. During this time, you are deaf and blind with regard to your own
senses.
If your familiar is reduced to 0 hit points, it will make death saving throws. If it dies, you take a -4 penalty to all attack rolls, saving throws, and ability checks. Every time the you finish a long rest, the penalty is reduced by 1 until it disappears.
Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.
When you reach 5th level, if your familiar is within 5 feet of you when you cast a spell that only affects you, your familiar gains the benefits of the spell as if you had cast the spell on it. When you reach 10th level, the familiar's Intelligence Score is raised to 8. Also, your familiar may learn and speak any one language that you know. If it has the capacity to, it can also read and write it as well.
See the spell description for more detail.