impedance matching for Guitar input
I am designing one pre-amplifier


if you are designing a pre-amp: then there are a couple of things you need to take in consideration


Hi-Z vs Lo-Z
Balanced vs Unbalanced
Line Level vs Mic Level


Usually (but not always)


Line Level inputs are Hi-Z (can be both balanced or unbalanced)
Mic Level inputs are Lo-Z (and usually balanced)
-20dB or -40dB pad's may be used for situations where line level is plugged into a Mic level input
Balanced inputs can be XLR or TRS
Sometimes someone plugs in an unbalanced Jack into a TRS input (you need to wire correctly for these situations)


(I just find it handy to have some DI boxes ready, knowing your outputs/inputs, and wire accordingly when the situation requires... much easier than catering for every possible combination)


  can some one tell me how the guitar inputs differ from line inputs ?


Generally speaking Guitar Outputs are "Line Level" "Unbalanced" "Hi-Z"
If the Guitar pickup/electronics are "active" (e.g. there is a battery or so)

If it is a Guitar without active components, then it is likely to be "Mic Level" "Unbalanced" and can be either "Hi-Z" or "Lo-Z" depending on pickup/built-in-mic.

Some "effects" used with guitar may increase even over Line Level, which means you need to attenuate, which can be done either by DI or on the Mixing Desk by "pads" or "attenuators" (terminology differs from brand to brand)

... my 2 cents ...

[EDIT]
As this question is migrated from sound.stackexchange.com to electronics.stackexchange.com, this answer is given from a "sound production perspective" without going into further electronics design. While I do have some electronic skills, I would not edit this answer further as I feel I would not be skilled enough to advise on electronic subjects like designing an audio-pre-amp to use with Pro-Audio applications. Please take that into account while you consider voting or commenting on this answer.