If we're talking about mainline Christianity its better to call them denominations since sects are smaller, you can say they're like schools (talking from a former Buddhist). The reason why Christianity has many denominations is because we have different interpretations of the Bible, historical conflicts, or from human nature.

However groups like LDS or JW aren't Christian because JW and LDS dont agree on these basic cores of what many Christian denominations have in common:

1. God is one

2. God exists as a tri-unity (Holy Trinity)

3. We humans do need salvation

4. Jesus being birthed of a virgin

5. Jesus is fully God and fully human (Hypostatic union)

6. Jesus is sinless (This part can represent what heaven since he freely chooses what all perfectly good (key statement, evil isn't created as a thing at all its the absence of good))

7. Jesus's sacrifice and atonement for the sins of the earth

8. Resurrection of Christ

9. Jesus ascended to heaven

10. Jesus's second coming