The deprecated ISAM table type will disappear in MySQL version 5.0. In MySQL 4.1 it's included in the source but not compiled anymore. MyISAM is a better implementation of this table handler and you should convert all ISAM tables to MySAM tables as soon as possible.
ISAM uses a B-tree index. The index is stored in a file with the .ISM extension, and the data is stored in a file with the .ISD extension. You can check/repair ISAM tables with the isamchk utility. See Crash recovery.
ISAM has the following features/properties:
Compressed and fixed-length keys
Fixed and dynamic record length
16 keys with 16 key parts/key
Max key length 256 (default)
Data is stored in machine format; this is fast, but is machine/OS dependent.
Most of the things true for MyISAM tables are also true for ISAM tables. See MyISAM tables. The major differences compared to MyISAM tables are:
ISAM tables are not binary portable across OS/Platforms.
Can't handle tables > 4G.
Only support prefix compression on strings.
Smaller key limits.
Dynamic tables get more fragmented.
Tables are compressed with pack_isam rather than with myisampack.
If you want to convert an ISAM table to a MyISAM table so that you can use utilities such as mysqlcheck, use an ALTER TABLE statement:
mysql> ALTER TABLE tbl_name TYPE = MYISAM;
The embedded MySQL versions doesn't support ISAM tables.