The arrangement of veins and the veinlets in the lamina of leaf is termed as venation.

When the veinlets form a network, the venation is termed as reticulate.

When the veins run parallel to each other within a lamina, the venation is termed as parallel.

Leaves of dicotyledonous plants generally possess reticulate venation, while parallel venation is the characteristic of most monocotyledons.

In the above picture, the first leaf shows parallel venation which is shown by leaves in monocots like maize, grass etc. and the second leaf has reticulate venation which is shown by leaves in dicots like pea, gram, etc.