Gin is reasonably easy to produce and the pungent nature of juniper can hide low quality distillate. There are two methods of making gin.  Cold compounding and distillation.

Cold compounded gins are neutral alcohol based, mixed with flavour essences derived from botanicals.  They are cheap and easy to produce, but produce unappealing gin.  All quality gin is distilled gin.

Distilled gin has the botanical ingredients added and redistilled with the spirit.  This can be done in various styles, some macerate, some brands seek for extra time and each process brings out different characteristics. The Botanicals, or flavouring ingredients, can be chosen from a list of around 50 with most brands choosing to use a blend of around 5-10 to produce their spirit. Common Botanicals include (in addition to Juniper): almond, angelica, cardamom, cassia, coriander, cubeb berries, grains of paradise, ginger, citrus peel, liquorice, nutmeg and orris root

Apart from various botanicals and distillation methods, gin is also defined very much by its alcoholic strength.  The flavours of gin will open up with the addition of water but as they are often at different strengths, that water quantity differs also.  Most are bottled between 37.5% and 47%.