The names of the strings are from lowest to highest:

E           A              D          G         B          E

The thickest string is lowest in pitch and the highest in pitch is the thinnest string.

The lowest string is an E string and is number 6, it is the thickest string.  The thicker the string the slower the vibration.  The highest or thinnest string is string 1 and is also an E string.

The strings are numbered low E6 – A5 – D4 – G3 – B2 – E1 high

Note names correspond to number of vibrations per second.  Octaves are equal divisions and for this reason the notes are given the same names.  For example:

A – 440     A – 220     A – 110

The numbers are vibrations per second or frequencies or hertz.

The frequency of the low E string is 82.406886 while the frequency of the high E is 329.63 Hertz.  Other instruments will tune to the same standard which is A 440, this sometimes varies slightly in different countries.  A piano tuner would use references like this to tune the piano.  When we use a digital tuner it reads the frequencies or vibrations and translates that to a note name.