Spectre, similar to Meltdown, is based on the ability of processors to do speculative execution.
Speculative execution is a technique used by highspeed
processors in order to increase performance by
guessing likely future execution paths and prematurely
executing the instructions in them.
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Meltdown affects mostly Intel and Apple chips and focuses on breaking the barrier between user and kernel mode, which can take place due to theses specific chip's privilege issues. Spectre affects virtually all chips since 1995.
Spectre is a flaw where program code can be used to trick the processor into performing instructions, with the help of branch predictions and speculative execution, it is not suppose to or be able to. Thus enabling user programs with very low or none privileges to access sensitive information via the memory of the kernel.
A branch in programming is an instruction that can execute another instruction and so forth which if visually represented would look like a tree branch. Branch prediction is when the CPU tries to predict which branch the next instruction will be in, before it knows definitely. Branch predictions and speculative execution walk hand in hand.
Updates and patches have been released, but because of the nature of the attack the patches slow down CPU time.However do not fear if you have a CPU running on Skylake, KabyLake or any new processor then the slowdown will most likely not be noticed. Older CPU's and people running Windows 7 or 8 will be most affected because of certain features of the operating system like kernel-level font rendering.
You can read more about Spectre here