TL;DRElectronically traded, smaller-sized versions of standard futures contracts. ES is the most traded futures contract in the world with millions of contracts per day.
E-mini contracts are electronically traded futures representing a fraction of a full-size contract. The most popular are equity index E-minis: E-mini S&P 500 (ES), E-mini Nasdaq-100 (NQ), E-mini Russell 2000 (RTY), and E-mini Dow (YM).
ES is the single most actively traded futures contract in the world, with millions of contracts daily.
ES: $50 per point, 0.25-point tick ($12.50/tick). At 5,200, one contract = $260,000 notional.
NQ: $20 per point, 0.25-point tick ($5.00/tick). Higher index level but smaller multiplier.
RTY: $50 per point, 0.10-point tick ($5.00/tick). Smaller notional, more accessible.
YM: $5 per point, 1.00-point tick ($5.00/tick). Smallest per-point value of the four.
Micros are 1/10th the size. MES is $5/point vs. $50 for ES. MNQ is $2/point vs. $20 for NQ.
Micros are extremely popular with newer traders. The tradeoff is slightly wider spreads and lower volume during overnight sessions. During RTH, the difference is negligible.
ES vs. MES for a small account
Your account is $10,000. You want S&P 500 exposure.
One ES at $50/point: a 20-point move is $1,000 (10% of account). Two MES at $5/point: same move is $200 (2%). Micros let you size appropriately.
Comparing E-minis
You're choosing between ES, NQ, and RTY for day trading.
ES moves 30-60 points daily ($1,500-$3,000/contract). NQ moves 150-300 points ($3,000-$6,000). RTY moves 15-30 points ($750-$1,500). NQ has the most dollar movement per day.
Starting with full E-minis before learning on micros
Micros let you practice with real money at 1/10th the risk. No advantage to full-size until your account and skill justify it.
Treating all E-minis as interchangeable
Each has different volatility, sector exposure, and daily range. They have distinct personalities.
Ignoring the overnight session
Significant moves happen overnight. Check Globex before RTH opens even if you only day trade.