A lot of people just buy products based on the way they are advertised, but don't actually look at the ingredients listed on the bottle - that's what matters, and is the most important thing to take into account when you spend your money.

I'm often asked what ingredients are bad in products, so I thought I'd do a breakdown of ingredients to avoid. 

Sulphates - sodium laureth sulfate/ammonium lauryl sulfate/SLS
Typically found in hand/body washes, shampoo, washing up liquid and often found in face washes, this ingredient makes the product lather. They are stripping to the skin, and give you that 'tight' feeling after cleansing. Switch to a non foaming, cream/oil/balm based cleanser instead.

Alcohol - SD alcohol, isopropyl alcohol, alcohol denat
Some types of alcohol used in skincare products aren't irritating to the skin, mainly fatty alcohols. The ones listed above are ones to avoid, as they are stripping and can cause sensitivity and redness.
Parabens - methylparaben, ethylparaben, propylparaben (pretty much any word on the ingredients list ending in 'paraben')
Parabens are used as preservatives, and have quickly become something that people have been choosing to avoid in their products. It is believed that there is a link between cosmetics (particularly deodorants) containing parabens and breast cancer, due to parabens being found in cancerous tumours. However, there is no definite evidence of this, so at the moment, it's personal preference as to whether you choose to avoid this ingredient. 

Mineral oil - petroleum, petrolatum, paraffinum liquidum
This is a distilled version of petroleum (Vaseline, basically). It is used in low quality moisturisers, serums, etc to make the product feel like it's moisturising...when it's actually not. It basically just sits on the skin and doesn't absorb, giving no benefit to the skin, apart from coating it and blocking pores. Have you ever noticed that Vaseline doesn't absorb, it just sits there? Yeah, you don't want to be spreading that all over your face.

Silicone - dimethicone, trimethicone, cyclopentasiloxane, cyclohexasiloxane (basically anything ending in 'cone' or 'ane')
This is used in hair conditioners, moisturisers, serums, to give a product slip, and to give a 'silky' feeling. Silicones remain on the surface of the skin, coating it, as mineral oil does above. This, along with mineral oil, is such a cheap ingredient to use in products, which is why so many companies use them, and is the reason why these products are so cheap.

*Tip - the higher up the ingredients list, the higher the content of the ingredient. For example; if you are looking at the back of a toner, and the top ingredient is alcohol denat, you'll know that the product has an extremely high alcohol content, and is mostly made up of that.