Most of any cosmetic product is infrastructure: things that hold water in, keep the mixture stable, stop it spoiling. Only a small part is the ingredient the marketing is about. Recognising which is which makes a label much faster to read.
Japanese names are given as they appear on the package, since that is what you will actually be looking at.
Water and solvents
水 (water) is the first ingredient in most lotions, toners and creams. エタノール (ethanol) appears in toners and sunscreens, where it helps a product feel light and dry quickly — it is also the ingredient most often behind a stinging sensation on compromised or sensitive skin.
Humectants — pulling water into the skin
グリセリン (glycerin) is the most common by far, and one of the most reliable. BG (butylene glycol) does a similar job with a lighter feel. ヒアルロン酸Na (sodium hyaluronate) holds a large amount of water relative to its weight. These sit near the top of the list in anything marketed as hydrating.
Emollients and occlusives — keeping water from leaving
スクワラン (squalane) is a light, well-tolerated oil common in Japanese skincare. ワセリン (petrolatum) is the heaviest and most effective seal. シア脂 (shea butter) and various エステル oils sit between the two. Silicones — names ending in メチコン or シロキサン — mostly provide slip and a smooth finish rather than a true seal.
Preservatives — stopping it going bad
Any product containing water needs them. フェノキシエタノール (phenoxyethanol) is the most common in Japan. メチルパラベン and its relatives (parabens) are still used and remain well studied. 1,2-ヘキサンジオール does double duty as a humectant and a preservative booster. These appear near the end of the list because they work at low concentrations, which is normal rather than a sign of a trace addition.
UV filters
Japanese sunscreens use both types. 酸化チタン (titanium dioxide) and 酸化亜鉛 (zinc oxide) are the mineral filters. The organic filters have long names — メトキシケイヒ酸エチルヘキシル is octinoxate, t-ブチルメトキシジベンゾイルメタン is avobenzone. Japan approves several modern filters that are not available in the United States, which is a large part of why Japanese sunscreens have the reputation they do.
Emulsifiers and thickeners
These keep oil and water combined and control texture, and they are the least interesting part of a label. ステアリン酸グリセリル, ポリソルベート followed by a number, カルボマー, キサンタンガム. Their presence tells you about the texture of the product, not its effect.
Antioxidants
トコフェロール (vitamin E) protects the oils in the formula from going rancid, and does something for skin as well. BHT does the same job synthetically.
Actives worth recognising
ナイアシンアミド (niacinamide), アスコルビン酸 and its derivatives (vitamin C), レチノール (retinol), トラネキサム酸 (tranexamic acid), サリチル酸 (salicylic acid), アラントイン (allantoin), and グリチルリチン酸2K, an anti-inflammatory extremely common in Japanese products marketed for irritation or acne. In quasi-drugs these are the ones declared as 有効成分.
Names that hide their contents
香料 (fragrance) can represent many undisclosed components and is a frequent cause of reactions in people who react to products at all. 着色料 or names in the form 赤色◯号 are colourants. Plant extracts ending in エキス are single line items that may be complex mixtures.
For how the ordering of the list works, see how to read a Japanese cosmetic ingredient label. nemi shows this per ingredient, in English, from a barcode scan.