When to use MyISAM and InnoDB?

Use MyISAM for very unimportant data or if you really need those minimal performance advantages. The read performance is not better in every case for MyISAM.

I would personally never use MyISAM at all anymore. Choose InnoDB and throw a bit more hardware if you need more performance. Another idea is to look at database systems with more features like PostgreSQL if applicable.

EDIT: For the read-performance, this link shows that innoDB often is actually not slower than MyISAM: https://www.percona.com/blog/2007/01/08/innodb-vs-myisam-vs-falcon-benchmarks-part-1/

Read about Storage Engines.

MyISAM:

The MyISAM storage engine in MySQL.

• Simpler to design and create, thus better for beginners. No worries about the foreign relationships between tables.
• Faster than InnoDB on the whole as a result of the simpler structure thus much less costs of server resources. -- Mostly no longer true.
• Full-text indexing. -- InnoDB has it now
• Especially good for read-intensive (select) tables. -- Mostly no longer true.
• Disk footprint is 2x-3x less than InnoDB's. -- As of Version 5.7, this is perhaps the only real advantage of MyISAM.

InnoDB:

The InnoDB storage engine in MySQL.

• Support for transactions (giving you support for the ACID property).
• Row-level locking. Having a more fine grained locking-mechanism gives you higher concurrency compared to, for instance, MyISAM.
• Foreign key constraints. Allowing you to let the database ensure the integrity of the state of the database, and the relationships between tables.
• InnoDB is more resistant to table corruption than MyISAM.
• Support for large buffer pool for both data and indexes. MyISAM key buffer is only for indexes.
• MyISAM is stagnant; all future enhancements will be in InnoDB. This was made abundantly clear with the roll out of Version 8.0.

MyISAM Limitations:

• No foreign keys and cascading deletes/updates
• No transactional integrity (ACID compliance)
• No rollback abilities
• 4,284,867,296 row limit (2^32) -- This is old default. The configurable limit (for many versions) has been 2**56 bytes.
• Maximum of 64 indexes per table

InnoDB Limitations:

• No full text indexing (Below-5.6 mysql version)
• Cannot be compressed for fast, read-only (5.5.14 introduced ROW_FORMAT=COMPRESSED)
• You cannot repair an InnoDB table

For brief understanding read below links:

1. MySQL Engines: InnoDB vs. MyISAM – A Comparison of Pros and Cons
2. MySQL Engines: MyISAM vs. InnoDB
3. What are the main differences between InnoDB and MyISAM?
4. MyISAM versus InnoDB
5. What's the difference between MyISAM and InnoDB?
6. MySql: MyISAM vs. Inno DB!