Solr Collection vs Cores

Core

In Solr, a core is composed of a set of configuration files, Lucene index files, and Solr’s transaction log.

a Solr core is a uniquely named, managed, and configured index running in a Solr server; a Solr server can host one or more cores. A core is typically used to separate documents that have different schemas

collection

Solr also uses the term collection, which only has meaning in the context of a Solr cluster in which a single index is distributed across multiple servers.

SolrCloud introduces the concept of a collection, which extends the concept of a uniquely named, managed, and configured index to one that is split into shards and distributed across multiple servers.

From the SolrCloud Documentation

Collection: A single search index.

Shard: A logical section of a single collection (also called Slice). Sometimes people will talk about "Shard" in a physical sense (a manifestation of a logical shard)

Replica: A physical manifestation of a logical Shard, implemented as a single Lucene index on a SolrCore

Leader: One Replica of every Shard will be designated as a Leader to coordinate indexing for that Shard

SolrCore: Encapsulates a single physical index. One or more make up logical shards (or slices) which make up a collection.

Node: A single instance of Solr. A single Solr instance can have multiple SolrCores that can be part of any number of collections.

Cluster: All of the nodes you are using to host SolrCores.

So basically a Collection (Logical group) has multiple cores (physical indexes).

Also, check the discussion