Terminator 1, 2, and 3 are direct sequels to each other, with Sarah Connor Chronicles, Salvation and Genisys being alternate possible futures.
In Terminator 1, Skynet was created and tried to destroy the humans, but John Connor taught the humans how to fight. Then, Skynet sent T-800 to kill Sarah, and John sent Kyle Reese to protect her. They destroyed the T-800, but his arm and chip were intact.
In Terminator 2, Miles Dyson used the T-800 arm and chip to create Skynet earlier than expected in 1997, and Skynet sent T-1000 to 1995 to kill John Connor, but John of the future reprogrammed a T-800 to protect his past self. Sarah and Miles destroy the Cyberdyne Building, stopping Skynet from being created and stopping the Judgement Day in 1997. T-1000 and T-800 are terminated with molten metal.
In Terminator 3, Skynet and Judgement Day were postponed until 2004 being created for US military, T-X is sent back to kill John and Katherine, and a T-850 reprogrammed by Katherine of the future is sent to protect John and her past self. They try to impede the Judgement Day from happening but are unable to.
Terminator Salvation: Marcus Wright appears and finds a teen Kyle Reese, who is captured by Skynet which already knows that he is John's father and tries to kill him to prevent John from being born. Marcus discovers he's a cyborg, and tells John he can get him inside Skynet. Kyle is rescued and Skynet Central is destroyed, Marcus gives his heart to John after he is wounded.
- was Skynet completely destroyed in this movie? will a T-800 eventually be sent back to kill Sarah, and then Kyle?
- why was Kyle locked in a cell, allowing John to save him, instead of being killed as soon as he was captured?
Terminator Genisys: John Connor knows Kyle since he was a child, so it's not the same timeline from Salvation. The Genisys timeline supposedly happens the same as in Terminator 1, but when Alex T-5000 attacks John, turning him into T-3000, the timeline branches off and Kyle has memories from two different timelines. A T-1000 is sent to 1973 (before or after John is attacked?) to kill 9-year-old Sarah, and T-800 arrives to save her. After many years, the original T-800 and Kyle arrive, but Sarah already knows everything that is happening. Pops deactivates T-800, T-1000 tries to kill Kyle, and then Pops kills T-1000 as well. Sarah wants to avoid the Judgement Day from happening in 1997 like in T2, but Kyle knows that it'll happen only in 2017 because of the timeline split. They travel forward to 2017 and meet John Connor, but Pops appears and reveals John is a T-3000. So and so happen and John dies and T-800 is upgraded.
- why does Sarah think that the Judgement Day will happen in 1997? does Pops come from the T2 timeline?
The Sarah Connor Chronicles: idk because I haven't watched it yet