The most direct (and therefore most reliable) way to get distances to stars other than the Sun is through the technique known as Annual Trigonometric Parallax.
A natural unit to use for the angle is a Radian. In this case, however, the distance to the star is given by
distance = (Astronomical Unit) / tangent( p ) ~ (Astronomical Unit) / p
The approximation that tangent( p ) ~ p can be made because p is usually a very small number. Okay, fair enough, but now what is a Radian? A Radian is defined by the fact that there are 2 pi Radians per circle. That is, we roughly have 6.28 Radians = 360 degrees or that 1 Radian = 57.3 degrees. When we measure the parallax angle in Radians, the distance to star comes out in whatever unit we use to measure the astronomical unit. Although Radians are the natural unit to use for angular measurements, in most astrophysical situations the angles with which we deal are tiny. They are in fact << than a Radian and a better way to measure angles is needed.
There are 360 degrees per circle. We sub-divide 1 degree into 60 parts which we refer to as arc minutes. We then sub-divide 1 arc minute into 60 parts which we refer to as arc seconds. That is, 1 arc second is 1/3,600 of 1 degree!!! In most astrophysical situations, the angles we deal with are on the order of arc seconds. The parallax angle for the most nearby star (other than the Sun) is around 1 arc second.
If we measure the parallax angle in arc seconds, the distance comes out in a unit known as a parsec. A parsec is the distance of a star whose parallax angle is one arc second. In terms of centimeters, 1 parsec is 3.1 x 10**18 centimeters. The smallest parallax angles which we can measure reliably are on the order of 0.01 arc seconds and so, we can reliably get distances to stars out to a neighborhood a hundred or so parsecs in size. This is only a tiny part of galaxy, the Milky Way galaxy, which is maybe 100,000 parsecs in diameter!