There are three types of inputs and outputs in a shader: uniforms, attributes and varyings.
Uniforms are values which do not change during a rendering, for example the light position or the light color. Uniforms are available in vertex and fragment shaders. Uniforms are read-only.
Attributes are only available in vertex shader and they are input values which change every vertex, for example the vertex position or normals. Attributes are read-only.
Varyings are used for passing data from a vertex shader to a fragment shader. Varyings are (perspective correct) interpolated across the primitive. Varyings are read-only in fragment shader but are read- and writeable in vertex shader (but be careful, reading a varying type before writing to it will return an undefined value). If you want to use varyings you have to declare the same varying in your vertex shader and in your fragment shader.
All uniform, attribute and varying types HAVE to be global. You are not allowed to specify a uniform/attribute/varying type in a function or a void.