If you look up “for” in a Russian dictionary, you will see at least two options — для and за. I will list some of the most common scenarios where you would use either preposition so you can grasp the difference better.

За + accusative

Support

За expresses someone’s support of a particular cause, candidate, or course of action.

• За кого́ вы голосова́ли на после́дних вы́борах? (Who did you vote for in the last election?)
• Пацифи́сты бо́рются за мир во всём ми́ре. (Pacifists fight for world peace.)

Exchange

За can also refer to any sort of exchange or specifically a monetary purchase.

• Я купи́л э́ти брю́ки за 4 ты́сячи рубле́й. (I bought these trousers for 4,000 rubles.)

Substitution

A related usage is someone or something representing someone or something else or acting in their/its stead.

• Во вре́мя мое́й боле́зни колле́ги выполня́ли мои́ обя́занности за меня́. (My co-workers fulfilled my duties for me while I was sick.)

Achievement

За is used to refer to someone’s achievement, especially is that person is being hono(u)red or celebrated for it.

• Мари́я Кюри́ получи́ла но́белевскую пре́мию за откры́тие ра́дия и поло́ния. (Marie Curie received a Nobel Prize for discovering radium and polonium.)

Cause

A similar usage is when за refers to something that caused the current situation, often when someone is facing the consequences of their past actions.

• За высо́кий рост его́ прозва́ли жира́фом. (He was nicknamed a giraffe for being so tall.)
• Бизнесме́н был осуждён за неупла́ту нало́гов. (The businessman was convicted of tax evasion.)

Для + genitive

Purpose/Destination

Для refers to where or what for something is used or will be applied. In other words, it introduces the “target” or a certain thing or action.

• У меня́ зако́нчился крем для рук. (I’m out of hand lotion.)
• Жа́реная еда́ вредна́ для здоро́вья. (Fried food is bad for your health.)

Objective

This is a related sense — this describes the objective of a certain course of action.

• Для повыше́ния у́ровня жи́зни населе́ния необходи́мы рефо́рмы. (Reforms are needed to raise the people’s standard of living.)

Recipient

Sometimes для is used to indicate the recipient of a physical object like a gift.

• Э́ти цветы́ для тебя́. (These flowers are for you.)

Perspective

Для can also mean “in the case of someone” or “from someone’s point of view.”

• Для поли́тиков поте́ря дове́рия избира́телей страшне́е пораже́ния. (For politicians, losing the voters’ trust is worse than defeat.)
• Для меня́ здоро́вье важне́е бога́тства. (Health is more important to me than wealth.)

I hope this helps clear things up a bit!